I would say keep it - it obviously has some happy memories for you and is an ideal size for your son to use. Also a relatively safe mechanism for a beginner.
These guns don't seem to make mega money even in vgc, from what I have observed on here, so selling it would be unlikely to make financial sense.
A modern replacement might be a HW30, although that is a break barrel as opposed to the underlever you currently have.
Happy Shooting!! Paul.
"We cannot solve our problems with the same thinking that we used when we created them" - Albert Einstein.