I was skinny because my parents didn’t take care of me not because I didn’t have appetite. In my teens I was 6’2” tall and 120lbs. Definitely lack of food will have some negative influence on your penis size. All my time I remember my penis was the smallest one and I’m sure that was because of lack of proper nutrition not to mention I wasn’t interested in girls and sex at all.
In country I live in before you graduate (19-20yr old) male students are supposed to have penis examination at 5yr old, 10, 12, 16-17 and 19-20. Those examination are obligatory so if you wouldn’t have them your parents would get fined or if you are 19-20 you would get into jail. I really hated them. At 19yr I had my last penis examination and I was so ashamed because my penis shrunk to something like 2 inches and examination was done by my neighbour who was a doctor and a nurse who was also my biology teacher . Every one was starring at me like I was a freak. It was 100 times worst than swimming in speedos in cold water.
At the age of 21 when I left my parents I started to eat and train and I have noticed that my penis gained in size too. I could feel the difference touching it with my hand. Unfortunately that was to late for my penis to gain some significant growth because my puberty was over.
So start eating, train more, get some extra testosterone doing testicles massage or injection and I’m sure your penis will grow.
In country I live in my penis is considered rather small but when I compared by measurements to global average I became very shocked. My length is 7 and girth 6 so it;s above average.