Like everything else, it really depends on conditioning. I do pony and cart rides with my minis and ponies. I keep my work animals in shape. My rule of thumb is no more than 1/3 the horse's ideal weight (For instance, my 32" mini--when conditioned--weighs about 200lbs--and when fat is closer to 225--I figure the 200) and that weight includes the saddle and pad so the child can be about 50 lbs. Your pony is bigger so you will have to get him in shape and then possibly take him to a feed mill or vet that has a scale and actually weigh him to know what you can do. It is too easy to over estimate. I will put up to 2 or 2 1/2 times his ideal weight behind him. When pulling a cart, I count the weight of the cart and figure this on fairly level ground. If you are doing hills or really grassy or other hard to pull areas, you need to lessen the load. Also, watch your horse for tiring and such. They will just go on and on and on so it is up to you to use good sense.
OOps, I just noticed it is a 'her' not a 'him'--sorry