I'm not expert in this, but it might be penile dysfunction. the most important thing he needs to see a doctor, if that's really bugs him. if he made the tests and nothing working, it might be emotional thing but I don't think so, you said you bought lingerie and i'M Sure you care about your looks and stuff, also that doesn't prevent it. good luck.
Obviously this is a pre-set response that's copy and paste or something considering it says MY BOYFRIEND and not me personally. And secondly hes 18 so feeling 17 again isn't any kind of accomplishment.
And im not buying him testosterone which can make him unstable and aggressive (ive a friend who uses it for their gender change)
please don't waste my time with answers that are obviously not going to help him