My son is a messy sleeper! Help?!

My son has recently moved to his cot as he's too big for his bassinet now! He's a great sleeper and sleeps all through the night! However in the last week that he's changed to the cot I haven't been sleeping! My son moves way too much in his sleep and gets himself into all sorts of messes and it terrifies me!! So I've tried tucking him in completely with the blankets all tucked under the mattress so he can't move all over the place, when I did this I found him under the covers!! If I don't tuck in the covers he kicks it over his head! If I don't give him covers he moves himself all about and ends up sideways with his head smacking against the rails! I tried putting him in his sleeping bag and this still doesn't restrict his movement!! He still ends up sideways! I'm at my wits end cuz I'm terrified something's gonna happen to him! Especially as he's almost 17 weeks and should start rolling over soon! Any help or advice would be appreciated!