Potty training

He has to be ready. He is very young still. I would wait till it's warmer and he shows signs of wanting to. DD never used a potty - a step and seat for the toilet were fine for DGS. We had them, DD had them as did other GPs. DGD only 2 but wants to sit on the seat - hit or miss if anything comes but no issues as not yet being trained. She just wants to copy big brother in everything so probably will not get potty either. There are a few books to help as well.

We always just leave their nappies off for most of the day and just keep gently reminding them about the potty.
Boys usually like the step up in front of the loo, then let him 'direct aim' himself. Great fun, I'm sure he'll get the hang of it.
For No 2 you can get seats that fit over your loo seat but with a smaller hole. They soon get used to them as well.
Most of ours didn't get on with potties too well but just try it out and then if not try the proper toilet with stand.
Most important is to not let them feel a bit of tension from you, it has to be fun/big boy and lots of praise, whoops and high fives.

If you feel he’s ready, I’d continue with plan to start training now. I found it was best to let them run around house with pants - no trousers. When sat on potty, run a tap- the sound of water running often triggers a pee?. Good luck.

I think it is easier to achieve in the warmer weather too.
He should be ready - do they still make potties that play a tune if success is achieved?

Why a timer
Lots of clapping and praise works well as far as I remember

Are his parents behind you with this? - because whatever strategies you start need to be continued at home or the lad will be confused by what is going on.

Like Cherrytree I think that summer is the best time to do this. I know it makes him a bit late with it all, but a few months is neither here nor there in the scheme of things - he has got the rest of his life to be dry/clean - and it would be loads easier for everyone.

Is he ready?
Use pull ups and leave potty in the same room.
Why not wait until warmer weather and then leave pullups off.

Not sure what you are going to use timer for?

Potty / toilet training is an on going process, will your stepson carry on with the potty training when they go home?

Imho patience is the key