It is a beautiful, and ironic, reflection of ourselves, when our skin veneer shows some wear and tear, but is more homely than any time before.
I always compliment my kids about how beautiful they are regardless of how awkward they may feel about themselves.
When my son complains about his lack of strength, "due to his noodle arms", I tell him, "Appreciate them. They will change before you know it, you will become a man, gain your strength, and become a new version of You."
Your son is lucky to have a mum like you. Isn't it crazy how even kids compare themselves to some ideal.