Well he probably has better genetics than you, or is eating better and recoverig better.
Your only real choices are:
- Train more - Sneak in some extra workouts, or extra sets, when he’s not looking
- Train less - Try 3x5 or 5x3(prolly not for dbs though)
- Eat more and sleep more
- Strengthen your shoulders
- Get a new partner
#1 will work if you aren’t currently doing enough or too much, but will fail miserably if you are doing too much
#2 will work if you are currently doing too much
#3,4 will ALWAYS help
and #5 is probably the easiest but dumbest solution.
Here’s what I would do. Since you said you are stronger at tricep and chest isolation movements, stop worrying about those, and focus on the presses and shoulder stuff. Dont completely cut them out, but put a lot less focus on them.
You said you are weaker on db press and shoulder press. So there are a few options here. You probably should start working both incilne and flat pressing more. The question is what angle for the incline. I could see vertical, 45 deg. or 30 deg. Any of those would be fine. And then keep the flat the same.
So to make it simple, go with 3x5 on incline, and then 3x5 on flat. You could even go 5x5 on the incline if you are very weak.
You may initially be lifting less when it comes time to flat bench, because you’ll be tired from the incilne, but when you get your flat back up to 42kg+ then you’ll be a lot stronger.