Hello, I am not a big fan of giving benadryl to toddlers as it can cause unpleasant side effects such as very dry mouth and either drowsiness or paradoxical agitation. If you choose to use benadryl, the dose would be 0.25-0.5 mg / kg of weight or 5-7 milligrams total. Recommend instead nasal saline to help him clean out his congestion.
I would much rather use the saline. Thank you for the helpful suggestion.