It might be worth keeping Sweden as long as they are a strategic ally and not trying to nab the province but outside of save scumming there's a good chance he never gets a PU/dynastic war opportunity. Plus even if he lands the PU, he can only benefit from Dalaskogen and whatever portion of Baltic trade Sweden controls by Siphoning income regularly.

He is far better off aiming to vassalize or conquer Sweden eventually. Conquest has its obvious benefits, with vassalization he can choose to get part of Sweden's income or make them a march, which isn't a terrible idea considering their military can be second to Prussia in troop quality. The only weakness to vassalization is combined power comparison to overlord.