Start him eating every 3 hours. Small meals. Don't let him eat or drink after 7 or 8pm, an hour or two before bedtime. I eat chili, pizza, heavy foods, things that take a LONG time to digest or that might give me tummy issues early in the day. Like before mid afternoon. Then for the evening meals I eat easily digested low carb higher protein meals that are out of the stomach and don't cause gas or acid stuff.
The smaller meals will digest quickly and then he won't have stuff in his stomach. Fluids on top of stomach acid make a volatile eruption. It can be crazy when you get horizontal.
You can put a 4"X4" block under each top corner of his bed too. If you can't raise just the head of his bed then find a way to raise his mattress on that end. They make foam wedges that go under the mattress or under the bottom sheet to achieve this too.
Laying flat is bad for reflux. It basically makes a pathway for the stomach acid to travel up and down from throat to stomach.
I would think that Pepcid isn't the right med either. If it's not working anyway. If his acid pumps made excess acid he needs an acid reducing medication, every day, forever probably. If he just gets acid indigestion when he eats certain things then he needs an antacid on demand sort of thing.