Baby’s delayed motor skills

So my baby is almost 9mo and he does not crawl, not even attempt it... he is not mobile at all, does not pull himself up, he seats well but only if I put him sitting on his bum, he cannot pull himself to a sitting position. He hates tummy time and any time I put him on his tummy he immediately rolls back into his back... what can I do to make sure he starts to move on his own??!!