Hi, I think if this was my child I"d probably pursue a less-and-less comfortable strategy until he chooses to sit in a chair. Begin to create a slope to your knees rather than dead level. If that eventually fails, I'd make my legs gradually jiggly and unstable. I'd probably also create a transition cue. I'd use a special dish towel on my lap for a few days before beginning the discomfort tactic. Then as my lap became less comfortable I'd offer to move it to a seat explaining with a shrug Mama's legs can't hold a big boy at the table. Maybe I'd drape it on the seat to start with and if he insists on the lap then, naturally, it's less comfy. Maybe I'd be less careful of my elbows and arms. A rocky ride.
I'd also do it all with great detachment. Tracy said about infants ... you can't calm a baby unless you are calm yourself. Still true for toddlers!
Alternately, it's easier to replace a habit than to change one. Change all the rules together. So perhaps everyone sits for dinner in a different chair than usual, or on a stool, or 'musical chairs' or something. Maybe eveyone gets a doll for their lap. Or a doll for his lap! IF you do everything the same, he will expect everything to be the same. Naturally.
As for him being slim ... well he just might be slim the end. I like this bit of advice: your job ends with putting good food in front of your child, then it's your child's job to eat it. You can't force him to eat and a child won't starve himself anyway. So relax when the plate hits the table!
best, g
