Hate to break it to you, but you cant teach him to vomit in the bathroom. At least I cant do it, I dont know about your son.
I remember when I was in second grade. I got up one morning feeling perfectly fine, I went to school, and as I was walking down the stairs I started to feel funny. I then stumbled against the wall, vomited, and fell over. Apparently I had a fever of 102. I had no idea I was sick, or going to vomit until about 3 seconds before I did so. Simply put, by the time my body tells me its about to vomit, I have 3-5 seconds before I start doing so. I'm 22 years old, and know full well the consequences of vomiting on the floor, but I cant stop my body's natural reactions. There simply isnt enough time to react and get to the bathroom. I think it has something to do with poor communication between the body and the brain.
The solution is simple: Carry a bucket.
If your son is feeling sick, just get a bucket and put it next to him (on his lap if possible). Then tell your son that if he feels weird, lean over the bucket.