This wont help really, but sometimes you see pics of people out hunting - er, exercising hounds, and there are some very 'round' gentlement jumping hedges etc , and I sometimes wonder what they weigh. I bet there are some that are 20 stone or more.
Assuming he's healthy, I wouldn't worry. [facetious mode] If the rider is not too fat to mount unassisted, he's probably not too fat to ride such a big horse. [/facetious mode]
*opens can of worms*
depends how chunky horse is, how old it is, how short and strong its back is, how much work it is doing (ridden for how long? at what speed? over fences?), whether rider is experienced or not, whether saddle fits perfectly, etc etc etc yadda yadda yadda.
taking all that into account, anything up to about 20 stone, possibly! maybe more if it's a mature strong horse going for a slow hack!
Not sure answer to above question, depends on rider, experience etc. I ride my daughters 15.2hh warmblood from time to time and I weight around 11 and a half stone, am told I ride reasonably ok, but not as well as daughter.
For interest, my own horse is a 14.3hh 16 year old mid/light weight irish cob. I bought him from a riding school who regularly used him for riding lessons. Because he is so good in the school and on the roads, he was regularly used by what they called 'larger, older first time riders'. Just before I bought him, he had lessons with a 22 stone first time rider who was trying to lose weight. No wonder when he was vetted the vet said his back was sore. I've had him for 5 years and he wont go back to that life ever. By the way, riding school was BHS approved.
some of those male showjumpers are no lightweights. Possibly the age of a the horse might make a difference, ie if it is a youngster and still growing, or if it is underweight and lacking muscle.