I agree with both of the above posts. Single rabbits can sometimes get bored and so just sit there, or it can go the opposite way and they can have sterotypical or destructive behaviour, either way a friend really helps a rabbit keep occupied.
It is also worth thinking about how much space he actually has, does he have the recommended 44sqft or over? Does he have this space at the times he would naturally be most active?
I also find that rabbits dont play with actual toys that much, and instead prefer environment enrichment like tunnels, different levels, hidey boxes, digging boxes etc. They also love things to destroy, especially when food is involved.
If you think he's being rather inactive, especially at times he should be active (dawn-early morning, dusk and late evening and the early hours of the morning I find mine are most active at) then you could encourage activity by getting him to forage more for his food, by hiding bits in hay and scatter feeding, and using a treat ball so he has to work for food.
If however your rabbit is sitting hunched and uncomfortable-looking, not wanting to move, and perhaps even grinding teeth - then this would mean he is in intense pain. It's likely not this though.