Mosquito bite swollen and painful?

My boyfriend was bitten last night by a mosquito. He is very susceptible to them, I've never seen someone get bit more often than him. Today he woke up and one of the bites is massive. It is bigger than a itched and aggravated mosquito bite usually is. It is extremely large, forming an obvious lump on his arm. It is red and irritating to him. He said his arms feels "heavy". We have put every anti-itch ointment we have on him and nothing is working. I know Benadryl is suggested but he seems immune to it. What can I do to relieve the pain for him?