Hi hun, Try tilting him back (in a bouncer is perfect angle), and using one hand to 'squish' his cheeks up, with one finger on each side of his face, so his lips go all pouty, like a fish. the give him a little at a time and work his cheeks with your fingers, squeezing them gently.

I hope that makes sense, it's hard to explain, but if you hold their cheeks 'in' they can't spit it back up.

HTH