I’ve faced this situation many times, especially when playing with people through Playtomic. Honestly, a player with that mindset can never be a partner for me; I’d only play with them again if they were my opponent Lol. I bet when you tell him to move up to the net, his response is either 'They are sending balls to me, how can I move up?' or 'Don't worry, I’m great at the back and I can return everything.'
Unfortunately, players like this miss the fundamental core of padel. To win, you must win the net first to put pressure on your opponents, that is where the real beauty and fun of the game. Changing this mindset is tough. he probably needs professional coaching to regain the confidence to play forward before he can be a viable partner again. For now, my advice is to look for someone else. Staying with a partner like this will only pull your level down instead of helping you grow. You’ll see others at your same level improving much faster simply because they have a partner who understands padel logic, proper positioning, and movement.