like the others said, its hard to get ur son to farm in chapter 3, do wat ramman321 said but this time marry marlin, ur son will automatically like farming but heres some tips to make sure ur son grows up and takes over the farm after you die.(must do these in ur sons first 2 chapters)
1) don't walk around outside use the horse
2) don't go off the farm, if u need seeds use the horse and quickly get over to Vesta's farm and buy them and get back
3) if ur worried it will take to long use the 99 of everything cheat , aka the cheat where u put ur controller into the third slot and press Z,
4)show him all the tools you have, don't ever show him stuff that doesn't involve farming
5) don't go in your sons room wen hes in his room, if u do and a cut-scene happens. say u want nothing don't ask wat is in the box! if u ask him wats in the box then u might as well just go befriend daryl (the crazy scientist guy) cause both ways make him wanta b something else...
6) always have atleast one thing growing in any one of ur 2 (or 3) fields it helps him realize you love growing crops and by chapter 3 (of your life, not his) he will water your plants if you wake up late, I suggest 10:00 am (if you don't have the alarm clock then go to Grants house at like 5 in the morning and he will give it to u)
7) realize that yes rock's son is super cute but he wont like farming
8) marlins son is cute and grows up to b a handsome young man that doesn't mind getting dirty, just doesn't like being told he smells