A relative of mine's daughter has cerebral palsy and cannot walk, but is mentally very able. She used to have a Safety First potty chair (the sort that looks like a mini toilet) in the lounge so that she didn't need to be carried far! She was able to ask to go and everyone helped her immediately. She was pretty reliable by 3yrs. They had no downstairs loo, so when she reached 5 yrs and was heavy to lift she used to use an Elsan basic camping loo in the hall with a childs seat on it!
I think that sitting on the potty at regular intervals to start with is a good idea, just to get used to it, and especially after meals. You may find that after a big drink, he may need to wee half an hour later, so you could try then.
If your DS can say "pot' or"wee'' or "poo" in time for you to help him, you just drop everything and dash. Some sort of potty chair close to hand at first, and when you know he can hold on for you to get him to the loo, you can use a child seat, and hold onto him, or put a chair in front for him to hold if he feels as though he will fall off!! Or you can buy a frame with arms from Argos p.1058 for £39.49 to put around the loo if you aren't supplied with one from the physio.
I would say that sitting with the knees higher than the hips is physiologically the best position for defaecation, so a foot stool might be a good idea.
I hope this will be of some use to you. Be prpared for lots of accidents to begin with, but you'll know you're both winning when you get more hits in the potty than in pants! Good luck!!