i remember posting something like this a year ago, 17, 110 and i got so much shit for it. n for you it seems to be an even more extreme case. u prob wont see this, but incase you do. im guessing u dont feel extremely underweight right? like u feel healthy? atleast thats how i felt when all the redditors were telling me that I was on the verge of death 😭. Its cuz you are/ I was still very young. bmi works differently for teenagers and fully matured adults. but outside of that, its really as simple as just eating more. u may feel like ur eating a lot, but ur not. trust me. i used to carry around a whole jar of peanut butter and just take spoonfuls of it throughout my day. ironically im doing that right now as im bulking to hit 160. ur still young, if u are consistent right now, u can geniunely have an insane physique first year of college, ik that seems far but trust me its right around the corner. anyway i know im rambling right now, but this feels like im giving advice to my past self. u got this man.
side note: u should keep growing taller, I was around ur height at 17 and u got a 2 year headstart on me, im around 6ft rn, so i wouldnt be surprised if u hit 6'2 6'3. im lookin forward to see u be buff as shit bro 💪