Hi, welcome to BW
Just as a quick starter, really at 7 months I'd start off with an A time of 3hr, keep that consistent (of course reducing a little to 2:30 or 2:40 if he takes a short nap) for a week and then tweak from there.
You have some good things going for you:
1. He *can* and *has* slept 1.5-1.75hr straight - this means he can transition from one sleep cycle to the next and that when you get the routine right, you should see at least one nap of this length each day.
2. You've got the medical issues sorted - that makes a huge difference as I'm sure you know
3. You've noticed heaps of things that can help us work out what's happening and how to help.
The 2 times I have tried getting him into a routine he has initially fought me but then responded well x 3-4 days/nights and then reverts back to being even worse than before
This is actually really normal - regression after 3-4 days is very common. It can also be a sign that the routine wasn't quite right if you're consistent through that regression and it doesn't get better within a few days to a week. Its a case of riding out the regression by consistently applying the same techniques you've been applying through those successful days.
He will often lie awake happily in his cot at nap time for up to an hour if I left him. Nights -he wakes at 4am and again is 'happily awake' and will stay there for an hour if I leave him to it
This long happy early morning waking is a sign of the A time in the AM not being sufficiently LONG. The thing that helps this is not sleep training, is not feeding or ignoring. Its keeping him up longer after he wakes for the day. In your case I'd say at least 3hr as a starting point. If he's not asleep but not upset, leave him to it.
You may be reading some of his boredom (get me a new activity, please mum) cues as sleepy cues. If in doubt, change activity and if still sleepy cues, then think about napping. FWIW, I would think you need to watch the clock so you're not watching for sleepy cues before 2:50 A time for him, otherwise you leave a huge amount of room for doubt.
General rules of thumb for nap lengths:
20min - overstimulation - TV + toys = too much for most babies. TV at all was too much for mine. Some babies need only 2-3 toys, sometimes only 1 toy at a time so they're not overwhelmed with choice.
30min - overtired - trim your A time a little or resettle (when a baby is used to A times far too short for them and you're pushing A times longer, sometimes there are OT wakings which you can just resettle for a couple of days and then LO sleep the long nap fine).
40-45min - undertired - not enough A time, bump it up.
These rules cannot be taken at face value on the basis of one nap, look for patterns through a few days before changing things otherwise you'll end up chasing your tail.