"my son is 6'1" and a skinny 150"
My son weighed less than yours and looked like a string when he first hit 90mph. So, gaining weight/strength won't hurt, but it's not the golden route to 85+. Technique is almost everything.
Velocity opens doors.
Likewise, my son was a true 6.0' and a skinny 145 and has always strove to be "bigger" (like when he was little after weighing in at the doctor's office at 43 lbs and telling the doctor of his goal . . . "I want to be 50 lbs."). As a HS freshman he was sitting at 79-80 and touching 82 (and keep in mind, this wasn't throwing all out as hard as he could, but more like 90% to maintain command). He got here because the year before we worked a lot on his mechanics and (in our case) never worked on strength and conditioning as I just felt it wasn't that important at that stage. He was already VERY athletic and had a strong and flexible core. His HS had a specified strength and conditioning facility and coaching that was mainly directed towards football, but baseball players would take part and later in HS my son got serious about that and the nutrition that needs to go along with it. By his senior year he grew to 6'2" and 185 lbs and his pitching was sitting at 90 and touching 93.
He was a two way player SS/RHP with most of his interest for playing SS. He was recruited for both way and turned down a draft as a pitcher to play SS at dream school college. As he entered college he felt his size still wasn't enough and he wanted to be "bigger", though he was touching 94 his college freshman year. His body had matured and as he increased his caloric intake and lots of strength and conditioning, height didn't increase but he got up to his goal of 205 lbs. in the sophomore year. IMHO, I don't feel that weight was good for him and it seemed to slow his agility down a bit defensively sprinting wise and he has a hard time maintaining any weight over 200 lbs.
After listening to and speaking with Tom House at a baseball coaches convention, he inspired me to focus on my son's mechanics. And I'm strongly convinced that mechanics/technique is indeed "almost everything" . . . particularly at these younger ages. I believe focusing on it early certainly worked well for my son.