DS is 5 months next week. He has rolled over about 3 times, the first time when he was about 6 weeks but has not started doing it consistently yet. I try to put him on the floor and on his tummny as much as possible but he doesn't like being in that position for very long plus he likes to be held so never stays on the floor longer than 20 minutes or so per session. Also, when on his play mat he tends to lay there sucking his thumb rather than playing like he used to! Should I be worried and what can I do to encourage him to move? He is very verbal and alert but just doesn't seem that keen to kick out and move around.