I Have An 11 Year Old, 80 Pound Son Who

I have an 11 year old, 80 pound son who has autism. I usually give him 3 mg of melatonin to help him sleep. Tonight he has a runny/stuffy nose and I want to give him a 25 mg benadryl pill for that, but I don't want the pills to interact with each other. He has already taken his melatonin pill for the night. In the future, is it safe to give both of these drugs on the same night?