Baby isn't motivated?

So I'm sure my son CAN roll to his belly, he's just choosing not to. He is able to sit without support now, he has started to wiggle like he wants to start crawling, but in terms of rolling.. nadda. He rolls to his side then rolls back. When I place him on his back, he just whines. On his tummy, he whines. I just want it to click already! I am tired of entertaining him for 90% of the time when he's awake. When he first wakes up he's inquisitive and does things to entertain himself. Then after about 45 minutes it's all downhill. I have to constantly put him in different positions, give new toys, walk around, put him in bouncers, we go to the store, etc., otherwise he whines and screams even if there's interesting toys all around him. Anyone else? Is there anything I can try to encourage him to get moving/exploring on his own? He legit isn't motivated to roll around, toys don't seem to do it.