Thumper’s diet, as with all rabbits, should be hay-based, at the very least 75% of his daily bill of fare and preferably 80% or more, with vegetables and pellets supplementary; in fact, some people prefer not to give their adult bunnies pellets at all but you do need to give Thumper alfalfa hay or alfalfa-based pellets for a few months more, up to about 6 months, then transition to timothy and other grass hays. I refresh my own buns’ hay twice per day, morning and evening, even if it’s just to throw another big handful into their litterbox and/or hay feeder depending on which needs refilling.
Is the lettuce you’re giving Thumper a dark green one such as romaine or red leaf/green leaf? Rabbits should not be given iceberg lettuce because it has very little nutritional value and can also cause diarrhea (if I remember correctly). Once Thumper is old enough to get properly started on veggies, you might give him a little spring mix and see if he likes it. Spring mix is composed of a variety of dark-green vegetables which are rabbit-suitable and a lot of house rabbit people, including me, give it to their buns for their daily salad.