Fifyfomum - a couple of thoughts. We used a potty that was more of a chair, with an insert in it, for potty training our three dses - I think it helped because it was a more normal sitting position than a potty - if you look at how they sit on a potty, it's not how you or I sit on a loo - and I don't think it is an easy position to wee/poo in. We changed from an ordinary potty, and the boys found it so much easier.
Another thing - if you can bear to leave it a couple of months, we'll be into the warm summer weather, and you could leave him to run around the garden near naked, and then you wouldn't have the worry of messes inside - that helped us too.
It might be that going to the loo somewhere where he can't sit and watch tv will actually help - because he will want to get the wee/poo over and done with so he can get back to his programme - at the moment he's got no need to do the wee, because he can see the TV where he is.
Children change really fast at that age - to you, September and school seem so, so close, and the potty training issue seems so urgent to you - but just because he isn't 'getting' it now, doesn't mean he won't 'get' it next week or next month, or maybe even in August. And your life will be so much simpler and more stress-free, if you can let go of these worries and concerns about him not keeping up with other children, and HAVING to master potty training RIGHT NOW.
Finally - please stop comparing your son to your friends' children - he is obviously not as ready to potty train as they were - but they all develop at different rates and times - it doesn't mean your friends did potty trainint 'right' and you did it 'wrong' - it means their children were ready to train at that age.
All through your child's childhood, there will be things he picks up at different times to other children, and there will pretty much always be someone who has done X, Y or Z sooner than him - but even if you don't know it, there will be children who haven't picked up X, Y or Z when your son has. Ds1 didn't learn to ride a bike without stabilisers until he was 6 or 7, whereas my friend's little boy learned when he was 5. Two of my dses had no problems with night time potty training, one was still wetting the bed at 12 - and yes, that was horribly stressful, and does help me have some insight into why you are so stressed - we had to give him medication if he went for a sleepover, cub camps and school journey were stressful and worrying for all of us, the washing machine was almost never off - and we knew that all of his friends - and his two younger brothers - were all dry every night, whilst we were still having to 'lift' him at 11pm, knowing if we forgot or did it too late, we'd be changing his bed again - and that we might well get woken in the night anyway, because he'd wet the bed again.
We had to stay calm, and not get cross with him, and eventually we found some ways of helping him (a bed mat wetting alarm 80% cured him, and eventually we just bit the bullet and stopped lifting him), but it was very stressful.
If you had tried potty training when your ds was 2, you might well have found yourself going through exactly what you are going through now - for nearly 2 years - because your son wasn't ready yet.
Nannies used to potty train very tiny babies by holding them on the potty until they 'performed' - and the babies learned to associate the feel of the potty with having to wee or poo - but it was reflex reaction to the stimulus - they hadn't learned to 'know' when they needed a wee or a poo, nor were they in conscious control of what they were doing.