Something similar happened to me but Muuuch worse. I ended in a relationship with a fuck buddy's best friend, and I didn't stop messing with him (the Fbuddy) untill I saw that the best friend really did like me and wanted something serious. I did this because the best friend just came out of 10 year relationship and I didn't think he wanted anything serious.
I told him everything about the situation before starting the relationship but 4 months after being BF and GF he started to feel insecure about the fuck buddy ( it was even worse because we all went out together sometimes)
Well it ended badly very bad. But I learned that dating an insecure person is bad for you and also for him. It only feeds to the need that that insecure person needs your approval for everything and it ends up controlling all of your life because he doesn't trust you.
My advise is, talk if not move on fast it's going to drain you. Build the trust with this person talking about everything and not leaving anything out. Ask him about his feelings about you hooking up with this guy, about you being an engineer and tell all the truth even if it's ugly. Help him understand your perspective and you understand his. Maybe tell him very carefully how you feel about his insecurities and that you want to support him because you love him.
But you have to understand that you can't give him the self confidence that he needs. That's on him to work on. He cant let that his insecurities brake the trust in the relationship. Relationships are hard and are even harder if you have to carry the burden of his personal issues. You can help but it's finally up to him to work it out. You both have to resolve the past and let go of it.
Personally I would let him go. ( In my case) His I insecurities finally ended up being mine, he always told me that I was a slut and didn't love him, that I had other boyfriends and wanted to be with the other guy and I ended up feeling like a slut all the time ( I certainly did something wrong but I knew it was wrong and wanted to improve, we all make mistakes, I didn't have to suffer so much for that) he also ended up controlling my life because he didn't trust me being with my friends or going to school and had to be with me all the time. I lost all my friends and stopped doing the things I liked to become only his girlfriend. I was blindly in love. Please don't be me. You have the right to have friends and be successful and he has to understand and support you. I cant tell you how happy I am now that I can talk to someone who supports my dreams and trust me enough to have friends.
Sorry for the long post, its just that I understand those insecurities and they are the worst. I wish someone would have helped me in that time. And also sorry for my bad English, I am just practicing again. Hope I helped.
This was super helpful. He still hasn't talked to me and he brought it up last night. I appreciate the long post and glad I'm not the only one that deals with this bullshit.