I too would let him help make the decision, but I would do it a little differently. I would invite him over to "look at the camper" as the first step of your trip. While the two of you are "touring" the camper he will get a true feel for the tight confines of an RV. If the space is a struggle for him I think he would immediately recognize it and bow out. The only preparation I would suggest is adding a pair of step stabilizers based on the rating of your steps. Some thing like this:
step stabilizer I've had three different guests in our MHs that were over 350 lbs and never had an issue.
If the two of you decide against him staying in the camper with you, you can always take a trip somewhere that has cabins that would allow him the privacy of his own space combined with the structural rigidity of a permanent dwelling. (as another member also pointed out)
Hope this helps.
Doug