when would it be safe to put a mainly flat pillow in the crib, my son is almost 10 months and loves pillows

DS has had a pillow since he was about 6 months. I know it is supposed to be not safe but it is very flat and he can't move it or hasn't figured out that he can yet. Basically the way I have his bed set up the pillow goes across the top portion of his bed. So it is just like an elavated mattress.

Just gave DD a pillow at 13 months and I am considering taking it out. She loves pillows on my bed but it just seems to get in her way in there.