It’s hard to know the ‘ideal’ weight for a bunny, especially mixed breed, but an experienced vet should be able to tell based on how bony they feel. You should always be able to feel some ribs and spine, otherwise they’re too fat, but they shouldn’t be protruding too much.
I think you’re right to reduce pellets again if it stops him eating hay. You can try and increase hay consumption by only giving him veg and pellets a couple of times a day, even if it’s a bigger portion it means there’s times when there’s only hay on offer.
If you determine that he does need to increase his weight, a good way of doing that is to supplement his feeding with a few syringes of critical care for a week or so. Normally, once a rabbit has reached a weight, they’ll then maintain it with their regular diet, so critical care is a good way of helping buns to put on weight while not interfering with their hay eating too much.
Lastly, is there a reason you only give him lettuce? It’s good for buns to have a variety of leafy green veg – try introducing some new things slowly, most buns love coriander/cilantro, dill, kale, mint etc. There’s a list of bunny safe veg in the BUNNY INFO section. Also veg won’t really help him increase his weight, so I wouldn’t increase it on that basis.