DS has been able to roll front to back since 5 weeks old. He learnt the other way at around 4.5 months. He's always slept so well in his moses basket, around 8pm-6am, and we transitioned to cot around a month ago at 5.5 months. Problem is, all of a sudden he wont stop rolling on to his front and trying to pull himself up on the bars, crawl, play on his front, etc etc... but won't sleep on his front. It's taking hours to get him to fall asleep! Is this a phase he'll grow out of?
Yes, this is a usual phase. You go through it when they learn to roll, again when they are learning to crawl (rocking on all fours in the cot). And you'll go through it again when pulling to standing.
I find the quickest way through it is to actively teach baby that in the cot he needs to lie down and that he should not be getting on his hands and knees or sityibg up.
To do this I'd stand by the cot at sleep time. Hand laid fairly firmly on his chest. Any squirming or trying to get up, re-lie him down and continue with the firm hand. Once he lies still and is calm, remove your hand but stay there. If he makes for moving return your hand to still him (re-lie down if needed).
It's consistency that will get you through this phase. Teaching baby to be still in order to go to sleep.
Just as you get it sorted, he'll start pulling to standing and you'll have to do it all again!