If he is gaining weight the laziness might not be lack of energy. Check if he is not feeling any pain. If he isn't I agree with Elizabeth on spurs or just insisting a bit more.
We can't put any more grain on him because he is gaining a lot of weight even on the light weight.
If you need to get them in front of your leg, try a spur. Also, try less pasture time or more grain to give him more energy
If he has a constant increase in his work routine, even if in the paddock, maybe think about increasing his feed, at least until he becomes conditioned to more work.