50th birthday present

Have you looked at notonthehighstreet? They do many personalised gifts, although I'm not sure if you're in the U.K.

www.notonthehighstreet.com/gifts/shop-by-occasion/birthdays/50th-birthday-gifts

For several members of my immediate family I have made a scrapbook containing the story of their life. I included mostly photos, either in chronological order, or in themes e.g. I titled a page Home Sweet Home and put photos of every home they had had. For my sister this entailed driving to the former homes and taking photos. I also included the bracelet that they wore in hospital with their birth details, certificates of achievement. When partners and then children came on the scene they were included. For one I titled each page with a song title that was relevant. I included lots of quotations e.g. Reader, I married him. The best part about those of my sons was that I relived those years, and they were both overcome when given them. In your case you will have time and postage restraints, but the result is a very personal gift, made with love, and something money can’t buy.

A bit on the lines of pensionpat's suggestion – but less demanding. Obtain a facsimile of the front page of a national or local newspaper for his date of birth and put it in a lightweight frame (reducing the postage). Check which looks the most interesting and choose that one. I did this for my father-in-law's ninetieth birthday and scored a bonus because the local paper that he had read and continued to read all his adult life also included the spectacular national news of the arrest of Dr Crippen.

giftitnow.com.au may have something suitable.
Or maybe some money for a special photoshoot for him and daughter together?

Look up Australian websites and find things there, they can then be dispatched to your son more easily and cheaply than posting. from the UK

Google Amazon Australia, they should have plenty of things you could choose from.

A visit from you ?

Since there is no other parent, IMO it would be a great "gift" if you teamed up with the teen daughter and helped her get something for him like the other parent usually would?

She could pick and you could finance and it could be from both?

Maybe fund a mini break for him and his DD somewhere near them?

Single parents often miss having that person who helps get their gifts from the children.

I have to say that I personally cannot stand not on the high street type trinkets and "on the day you were born" type prints. And because they're personalised its a waste because no-one else can use them.

Is there a gig or a sport event that he and his DD would enjoy?

I haven’t done anything different either for my own or my children’s numbered birthday I don’t think any number is different to any other... obviously just me

In Aus I know they love going out for breakfast. I got in touch with several cafes and paid up front for breakfast for 2 and then sent my Bil vouchers to use.

I always ask for a present list for birthdays and Christmas and choose from that. Buy a small present from the list and the rest is money.
The thought of paying good money out for something a person does not want is a waste.
We all do the same in our family and it works a treat.
My MIL who is now dead didn't like this idea and most of her presents went straight to the charity shop.
She loved a bargain but my worst present I received was a pr of shoes which didn't fit.

Thank you for all of the suggestions. Apologies for not saying thanks earlier. There'll be something there to grab my imagination.