I really don't like the Rocks Off prostate toys, so I'd avoid the cheeky boy. They have such an awkward shape, so before you buy it you have no idea if it's going to fit his inside body properly, and you don't want to get something which will turn him off the whole experience. I also find the bullet that powers them to be disappointing, by the time the vibrations travel through the length of the toy they don't feel very strong, and the single function isn't very stimulating.
I'd suggest the Lelo Billy, it's on the more expensive side but it's worth it. It isn't very thick, which is good for someone new to anal play. The vibrations are strong, and adjustable in intensity. It has several patterns of vibrations to choose from, which gives you the fun of variety, and the chance to pick one which works the best. It also has a handle, which means you can hold it and physically use it on him, while he just lies back and enjoys the ride. For a bonus, if he doesn't like it then you have yourself something which doubles as a great vibrator, so it won't go to waste.