Play outside. I took our "training potty" outside on the deck and the kids played in the yard. They wore soft shorts with no underpants at first or just underpants. I kept "reminding" them to stay dry. We'd just dump it in the garden or inside toilet as needed. I also kept a small water jug with a spout outside with liquid hand soap for them to use afterwards, and a towel on the railing.
Remember after meals or with lots of drinking you may need to encourage him to use the potty more frequently. You can give him a special little toy that's ONLY for when he's on the potty.
If and when there are accidents, just remind him again to keep trying and don't make a big deal out of it. Maybe if he can "go" all morning without an accident, he can have a "treat" (like a dum dum sucker, bag of fruit snacks, etc.). My kids LOVED the ice pops you can buy in a big box at Jewel/Dominicks/WalMart (not frozen yet). He can get a "pop" as his treat for staying dry.
At this point, you'll still have to remind/encourage him to take breaks to "go". He doesn't know that after breakfast, lunch, etc. that he'll need to. Sometimes I'd tell my kids, "Boy I need to go potty! Let's take a break and get a snack, ok." (We do the same thing if we're at a restaurant, store, etc. to stay dry on the go.) That way EVERYONE stops and he doesn't miss out on any fun or feel isolated.
I agree that too many layers of underwear and pants are too complicated right now. We used pull ups at night and again, had them go before bed, sometimes woke them up before we went to bed, however the BIGGEST problem we had was at about 4-5 am. They just couldn't hold out.
My cousin suggested "remaking" the bed for this stage - putting down a fitted mattress sheet, top that with a waterproof one, then make the bed like usual. Then, when there are "accidents" in the middle of the night, you can strip off the regular "sheets", including the waterproof pad, and then you have a bed ready to go. This "trick" helped us to keep the lights low, commotion low and back to bed quickly.
During the daytime then, we stuck with training pants and brought extra clothes everywhere. I ONLY used pull ups for long car rides and overnight. Also, we put "pee pads" in their car seats. These are those smaller sized adult incontinence pads/pet training pads...plastic on the bottom,and absorbent top layer. Again, these were more for accident protection.
I think overall the key is to let him know to do his best. And make a BIG deal out of little milestones. When accidents happen, ask him why it happened. We learned from my littlest one that "pull ups are for catching the pee pee". No matter what we said, she was convinced they were "big kid diapers" made to be peed in. Hence the reason we didn't use them much.
Good luck. I hope this helps.