Suggest Treatment For Lump On Shaft Of Penis

I noticed a very small lump on the shaft of my son s penis several months ago. It did not hurt, was not discolored, nor was it hard. His father said it was likely a small scar from when he was circumcised which i may never have noticed. Today, he said his penis hurt, and when he showed it to me, It is very red and swollen. It reminds me of a big zit. He is 3.5 years old. I do not know what it is or what to do. Should i treat it as a pimple and apply a hot compress???? I am a little freaked out.