Startle Reflex When Flat

My LO is almost 10 weeks old and we are trying to start transitioning out of the rock n play for his night sleeping. However, he hates the swaddle and he ends up startling himself awake when sleeping in the bassinet. I've read about the magic merlin sleep suit but we live in Texas and I worry he'll get too hot since he's warm natured anyways. I've got the halo sleepsack/swaddle coming today so at least he can move his feet. Just wondering if any other mama's had suggestions on how to get him to sleep on his back (or how to calm that startle reflex) and hopefully sleep for longer stretches.

Thanks ladies!!