He's not ready to go. The more you focus on it the more he's going to make it an "I'm in control" issue.
I'd let him wear his pee/pooh pull up for a while so he can feel how gross is it and then maybe he won't want to wear it anymore.
I don't stress over potty training. It's down on my list of absolute things that have to be done by such and such a date.
We gave the kids 1 M&M for pee and 2 for pooh. Sometimes we did Jelly Beans or some other small candy. It worked better for us.
What kind of bed is he sleeping in if not a big kid bed? He's way too old to not be sleeping in a big bed. Baby bed mattresses are not meant for older kids and they are darn uncomfortable for bigger kids.
As to the meds you are giving your kids.
Sorry, your doc is nuts. There are much better meds that have way less side effects than what you say you've giving him.
Benadryl dries their sinuses and throat and lungs out horribly. It's job is to stop mucus from being runny and stop an anaphalactic allergic reaction. It is a serious allergy med and NOT for making kids sleep. I'd change docs today if ours said to do that.
There are about half a dozen meds that are NOT habit forming and insurance will cover them that will not make his sinuses bleed inside. That's how drying Benadryl is.
I know this isn't what you want to hear but I can't imagine giving a kid an allergy med to make them sleep.
The number one side effect of allergy meds, other than sleep, is behaviors. The kids act out because they are IN PAIN from the dry sinuses and sore throat and dry lungs. It's painful for them.