1) when you put him in put his feet in first so he knows what's coming
2) Make sure you're holding him securely
3) if you're using a baby bath make sure his feet are touching the end, it's all down to how safe they feel
4) using a flannel/cloth wet it with the warm water and place over his tummy, like a blanket, and then using a sponge keep squeezing warm water over the flannel - again it's all down to them feeling safe.
5) i would always talk quietly and calmly just so my lo knew this was a nice time..... the squealing and kicking happens now!!!
we did all of the above for a good few weeks and our lo has loved his bath from day 1... i hope this helps... let us know how you get on x