Hey D.! My daughter is now 13 1/2 months and she sleeps with a blanket for warmth.
Since she was a baby I've put a blanket over her but she was right by me in her bassinett for the longest time so I could keep a watch on her with it (for safety). Then as she got older I used a blanket sleeper which worked really well. I just put on her warm jammies & put her in the blanket sleeper & it was great. But she soon outgrew the one I had & I didn't replace with a bigger one.
So now that she's older I started just tucking a light blanket over her in her crib. I made sure it was tucked in nice and tight on the sides of the mattress. So far having a blanket in her crib hasn't been a problem for her, even when she moves around alot. (she wakes up alot at night so I end up checking on her quite a bit - she usually ends up in bed with me anyway at some point in the night) But during her naps she does really good with her blanket.
Hope this helps. Just remember that all kids are different sleepers so maybe a blanket sleeper over his warm jammies might be best for you since he moves around alot. Good luck!! :)