try to analyze your games after you play and play higher time control (like 15+10) to really get better at calculating
How to break past 1700 elo rapid.

calculating isnt really the problem if he's 900+ points higher - work on endgames and converting advantages, as well as saving half points from worse positions. this can help since you can attack knowing its not nessacarily lost if it goes wrong. I'm not really qualified since I'm only 1850 rapid, but thats only bcs i rarely play it - I'm 2100 blitz and 3200 puzzles (1100 points higher because im a tactical kid)
i mean he can work on the endgame but at 1700 i think he should first work on middle games, and i think also work on tactics bc 2600+ is kind of low at his level
I have 2600+ puzzle rating and review every game after, however I'm still stuck around the 1700 elo area.