16.5 downlane isn't extremely fast and 275-300 revs aren't that low. With a Proton and Exotic it seems odd that he doesn't see enough response off the spot. Unless those balls are way too strong for the conditions/where he's playing or his release is very much up the back.
Thinking about this a little bit, I'm thinking since the rev rate isn't that low with average to lower speed and a THS, the ball might be burning up downlane. The Proton comes at 2000 grit OOB. That is quite a bit of surface to work with. Mix that in with having a lower RG core and a medium-high differential and the ball is designed to have a little length but still be early overall. Then the Exotic has a low RG core and high differential with a Nano cover. This means it will still be early.
My thought is that if he wanted to stay with asyms on THS that he should look to surface changes first and probably add some compound or polish. This will let the ball skid more through the fronts and help store some energy for the back. Another option potentially is to look at weaker layouts. In either case, try surface changes first as that is one of the most important things to look at. The lower the RG value on the core means that the earlier the hook. Higher differential means higher flare potential. This combination on THS can be tough to use on a consistent basis.
If the balls simply don't match up, I would look more towards a symmetrical solid/pearl/hybrid and then adjust surface as needed and/or use in combination with a pin above layout based on PAP. Also, I would potentially look at moderate to higher RG numbers for the core and then medium differential as this will help give easy length, store enough energy to react downlane, and not overly flare.
Balls that typically fit this realm would be the Storm Phase V, Storm Hy-Road, Roto Grip Idol Cosmos, Brunswick Knock Out Bruiser, Hammer Dark Web, Columbia 300 Cuda, Motiv Ripcord to name a few.
These are all general suggestions. However, I would encourage him to talk to his PSO to go more in depth.