Throwing Up - Salt Lake City,UT

Might be a very mild stomach bug (is there any fever?) or maybe if the weather has been very hot it might be affecting him a bit.
A sudden hot spell can make me feel a bit sick to my stomach even if I'm in air conditioning all day.
I'd try putting down a towel with a plastic bag or shower curtain under it next to his bed for a few days so clean up is easier.
Try making his last snack/meal before bed happen a bit earlier so it has more time to get past his stomach before laying down for bed.
Keep him well hydrated.

I agree with one of the other posters that it could be drainage. He could also have some reflux going on. It's possible for kids to have it without them or us knowing, especially when they're not babies.

? sounds like reflux / acid stomach?

Try having him sleep propped-up on pillows?

Could sinus drainage or post nasal drip be causing him to cough or gag when he lays down, and making him spit up just a little? When I have a slight cold, or allergies are bothering me, when I lay down it shifts in my head and can make me cough, and sometimes I gag too. It wouldnt make him feel sick, so he might not think to get up and tell you.

My four year old son does the same thing.

I can't figure it out either.

Maybe a growth spurt?