No, it's not that easy to tell since he's lying down, but although I can see some rib outlines, he doesn't look seriously skinny to me. Very cute face too
Well, it's really up to you if you want to give the satin balls a try. If you do, just feed one meal a day of satin balls, and keep the other his regular food (I'm assuming he eats twice daily - but if it's still three times, then I'd keep two meals as regular food). If it's a bit much for his stomach and he gets the runs or throws up, you can always revert to the regular food. A couple of weeks of that regime is enough.
As a longer term measure (or just alternatively) the addition of healthy raw or lightly cooked meats to his food could certainly be beneficial

Good luck, anyway, and let us know how he gets on.
Gwyneth
PS: I think you posted a couple of pics of a white boxer as well. Those weren't approved because they were too small. You can upload them again though, just a little larger please

A minimum of about 200x200 pixels is needed for gallery pics, but 640x480 is the best size
