To my knowledge, tomato plants are only toxic if ingested. So technically, sitting on the plant wouldn't be toxic, but if he takes a bite OR zaps a bug that's been eating the tomato plant, it IS toxic. This is the reason you can only feed chams hornworms that have been feed an artificial diet, not their natural diet, which is tomato plants.
It seems quite risky IMHO to put him on the tomato plant for sun. Perhaps bring a safe house plant out for him to sit on?