Give him as much as he wants, yes you can plant the seeds. Do some reading on cactus growing you don't water them like other plants and temperature will be a factor to get it to germinate.
My babies love it! The seeds are pretty big and hard as little rocks. At first I'd feverishly pick them all out and serve the prickly pear "mash" they loved it ofcourse. Now I don't bother with the seeds just cut the fruit in sections (my babies' mouths are still very small) and put down on top or mix it in with their food. They love it. You can always tell during bath when they ate cactus fruit, there are tons of little etched seeds in their poo
I can't get ours to eat these or any haubisicus leaves/flower but as someone else pointed out they are extremely easy to grow. I ordered 5 rooted pads on eBay for 10$, I'm HORRIBLE at planting, growing or replanting plants but these have thrived just stuck in a pot these are only a few weeks old but you can see new growth(I haven't cut off yet)
It could be because I cut them lol that's why the other tops have cuts? Or maybe i didn't cut that one in not sure but normally all my plants die in a week