How can I acknowledge all my eldest grandson has achieved without making his younger brother feel left out?

Can you not praise both of them ?
So the eldest is growing into a lovely young man - the younger may never be the same.
They are different boys, with different personalities and they both need to feel loved.
I really don't understand why the younger should miss out because he is not, at this moment, conforming to your ideal.

Mamma 66,I know how you feel, I have two lovely Grandsons, the eldest is starting University and gets a lot of praise from everyone,his younger brother has been unfortunate he was ill last term and didnt pass his exams as was hoped,I feel so sorry for him when we are all together and everyone is talking about Uni with the older boy, I feel the younger one, 18 years old, must feel a bit left out but wouldnt show it, I make sure I praise him too for his acheivements,I treat them equily,I wish others did.

I do agree with Bluebell your post comes across that you favour the eldest. I’d tread carefully if I was you. Lots of good advise already given, if it was my GC I wouldn’t do it personally.

Well thats very simple - encourage them both and nobody will feel left out....

Surely, you can find something to praise in the younger child, too?

I have never known a child, and I have taught hundreds, who didn't have some good or kind trait or one subject he or she was good at and could be praise for.

Please find reasons to praise the younger child, or stop praising the elder.

There are loads of things that you can find to praise even the "trickiest" of children. Even when they have done something incredibly naughty you can encourage them to make things right and give lots of praise for that.
My mother was totally and absolutely besotted with my eldest to the exclusion of all her other grandchildren and we are still picking up the pieces. The other grandchildren were hurt but bounced. The one she favoured struggles with her relationship with them, has no respect for my mother, felt marginalised by me because I was always trying to redress the balance and has struggled to live her own life without resorting to going to my mother to rescue her with handouts.
There is good research to show that it is the favoured who suffer the most so don't inflict that on a grandchild if you love them.

Where is that research, icanhandthemback?
Most parents are careful not to treat their children differently, but most parents do have favourite children, even if they would not admit it openly.
And in my experience, a favourite child does feel secure and loved in a way that sets them up for life.

My two grandkids are a boy 13 and a girl of 8...... I was alone with eldest for five years and our relationship is wonderful . The little one is a totally different personality .... but I love her to bits anyway.... although she can be a trial. I like a challenge and she gives me one. Both get the same for birthdays / christmas’s, and both get told how much I love them .... which is no lie. I suppose because the older she gets the more like me she gets.... it’s irresistable to see yourself re-born. Doesn’t matter how I feel about them at any time in their little lives, they will always get same praise and same rewards.

....you can surely find something positive/good about the youngest

I made a fatal error yesterday when meeting my DD and DGC for lunch completely forgetting myself I hugged the older one as she surprised me when I got out of the car! She instantly turned round and said to the younger one “see I told you I was nanny’s favourite “ I spent most of the the lunchtime reassuring the younger one I loved her equally as much (which I do) as the older one and not to take any notice of her sister! I agree if one has had achievements at school it is not wrong to reward that as a GP as long as the other child knows they will get the same when their turn comes around. My eldest DGD had 8 distinctions in her latest exams so I gave her a monetary reward for studying so hard. The next day I bought the younger one a new pair of trainers she wanted/needed. Both happy - result! You can always find something praiseworthy to spoil your younger DGS with so he doesn’t feel left out and second place to his brother.

We started how we meant to continue with our 4 grandchildren . They do not all get something if one is getting something for a special reason BUT they do all get something over time if you see what I mean.

We have created an understanding that each is treated as an individual not a ‘pack’ of grandchildren ?. So there is now no expectation because A received something that B,C or D will get something at that particular time.

We do praise individually if it’s warranted and no one takes offence because they know when they do something excellent it will be there turn.

Not much help to the OP I know because this is what we have always done but I do feel if the eldest child deserves praise he should get it. The younger child can be praised when he does something too.

Quite agree N&G I dislike the idea of children feeling entitled to get something just because another child does.
I knew someone who measured the drinks in her children's glasses to ensure they both had the same amount, they had to have the same food on their plates etc. Ridiculous.

Exactly what I said earlier! Everyone is so set on there being a favourite with the OP, but I just don’t see it ?.

Be very careful how you approach this. I have 2 boys and the first born was favoured by his father and my mother and my mother didn't hide the fact that he was her favourite. At the age of 8 my youngest said "why does nana X like him more than me?" How do you answer that or try to make up for it without being accused of playing favourites yourself. I found this extremely upsetting.