How do I keep my 3mth old's limbs in his cot!!?

I wake up to my little boy in the middle of the night for a feed and every night in the last week, his legs are hanging out between the bars of the cot!! He must wiggle a lot in his sleep as no matter where in the cot I put him down, every night its the same thing and he cant get them back in by himself. I have heard not to use cot bumpers as they are a suffocation threat?? What do I do??