You definitely don't want to have him using his bottle while in a lying position. It can lead to ear problems/infection because their systems are so immature that milk can back up into the ear.
Why not try giving him a spout type sippy cup and see what he does. I started giving my daughter a sippy at around 6 months to get her used to it. In the very beginning I took out the inner spout and put a tiny amount of my milk inside so she could understand what it was for.
And the AAP is recommending that parents get baby off the bottle and to a sippy cup by around 9-10 months to prevent extended use of the bottle (and bottle fights when baby needs to give it up lol)