Heavy Metal (1981) is often compared to Fantastic Planet. There’s no son, but lots of the other things you listed match.
https://en.m.wikipedia.org/wiki/Heavy_Metal_(film)
https://m.youtube.com/watch?v=yKkt_g9Tc7M
https://m.youtube.com/watch?v=T6a3F6fPSP0
https://m.youtube.com/watch?v=dFq0cHHYrSU
Or maybe Heavy Metal 2000 (2000) https://en.m.wikipedia.org/wiki/Heavy_Metal_2000
I’m not the OP, but I couldn’t leave y’all hanging with no moss.