Thanks for your question and concern as you said that after sex the penis of your boyfriend becomes sore and goes red then it is sort of infection but don’t say that the pain has started recently or is it going on for a longer time if it is so that his pain has started recently then I would suggest him to get a visit to his nearest doctor or a doctor at genito urinary medicine for a check up because in medical terms this pain during sex can be the chances of having Sexually transmitted infection. And as you said that you went for the checkup and you were given allclear but you must have not got the full screning during your test so I would suggest you to get yourself tested again. Few STI can’t be detected easily. Behind all this there are the possibilities that your boyfriend is facing several other pain but my suggestion is to get it checked in order to know about what really it is.As you told me that you both are too vigorous during sex and you are using condom then might be your boyfriend could be allergic to latex in them. According to me I only find a way for your boyfriend is to get him tested.As well it is for you also to get yourself full screening for your asurety because in this case the chances of STI are very high.I hope I have cleared your question.Take care