My DS is 14 weeks and is rapidly growing out of his 0-6 month sleeping bags. He loves kicking his legs and doesn't really have enough room to do it in his current sleeping bags. Would it be ok to move him into the bigger size early or is it a SIDS risk?
Look for one of the ones that fastens at the neck. I have 2 6-18 month ones I bought in Mothercare. One is a Grobag that fastens down the front, so has a fixed size head hole and would probably not be good for your son. The other one is a Mothercare own brand one, and it has two sets of poppers to fasten it at the neck. So, in other words, you can vary the size of the head hole.
That said, DS is a big boy with a large head to boot (top centile for length and head circumference) and when I put him in this sleeping bag at 5 months he was only just not able to pull his head in it...
If I were you I'd stick it out with the 0-6 month bags for a little longer if you can.