No easy answer for augmentation on athletic build

Unfortunately, this is a compromise operation. You will have to accept some of the disadvantages that are naturally a part of the operation and some of those are emphasized by the fact that bodybuilders in general have a very low percentage body fat which makes visibility of the implant much more likely. This is true whether the implant is placed under the muscle or over the muscle.   One issue to consider is size. Going too large will not only give a more unnatural appearance it will increase the chance of implant visibility meaning a greater chance of visible rippling and wrinkling. A more conservative size implant will look more natural and that will have less of the chance of visible rippling and wrinkling. A more conservative size implant will also keep you looking in proportion and more athletic.  With regard to this issue the silicone implant is preferable because in general it not only feels more natural but it looks more natural because there is less of the chance of rippling and wrinkling compared to saline implants.  

The next issue to consider is whether to put the implants under the muscle or over the muscle. There are disadvantages to each of these choices in bodybuilders. Over the muscle or more accurately, sub glandular, results in less soft tissue coverage which could increase the chance of being able to see the implant. This means that rippling will be more visible.

Unfortunately, placing the implant under the muscle increases the chance of movement of the implant with contraction of the pectoralis muscle, which in many cases is undesirable. It can be difficult to predict how much movement of the implant will be present, but in all likelihood, there will be some. So you need to choose which of those problems you want to try to avoid, and which of those problems will be acceptable to you if they are present after the surgery.

I would recommend silicone gel implants and leave up to you whether he would prefer over the muscle, sub glandular or under the muscle, sub muscular given the issues noted above.