My tips - no idea how helpful this may be as they are all different, but its worked for me twice -
Sounds like he's still under the weather, so don't worry about pushing anything until he's well over it.
He's also still relatively little, some kids don't get the hang of it until later, so don't worry overly much.
Definitely don't let mealtimes go over, say, half an hour (unless they are having fun of course). Call it quits after that otherwise you'll drive yourself crazy.
Look at the amount and timing of milk feeds. Concentrate on introducing one meal time at a go, don't even think about 3 mealtimes until one is going quite well. Eg, milk for brekkie, solids lunch (maybe about 11 if hungry by then) and small milk drink after, then milk in the afternoon and tea/bed.
If you're giving water with solids, try not to offer it until at least half way through.
If you think that there might be a negative association with highchair etc (or just stress vibes coming from you), then try for a picnic lunch for a couple of days. Yes it will be messy, but it might relax things a little, and if you're outside it can't get too messy (ha ha, famous last words - I particularly recall my brilliant sardine sandwich picnic idea, everything smelt of fish for days)
What are you feeding him? Does he prefer smoother/lumpier food? Hotter or colder? (both mine like their food almost cold). Sounds like you offer finger food, I would definitely always do this (e.g. to start with some rice cake or organix carrot puff whilst I gave a couple of spoonfuls of mush by subterfuge, later on could just give sandwiches or pasta etc). I imagine you've tried all the permutations though.
If traditional weaning just isn't doing it, maybe just do baby led weaning - give him a few options and don't fret if he doesn't eat them kind of thing. Might take longer to get him eating more, but could be a lot less stressful.
Sorry - most of that probably not very helpful, but I know how stressful it can be to start with so hopefully something helps, if not just the reassurance that it is largely down to them and not you (DD1 was a bit hard, DD2 was a human hoover!) and you can only go at their pace really.