Bf hardly ever finishes

My bf and I hv sex around twice or 3 times a week and Sometimes after going at it for around 2 hours or something he just doesn’t look as upto it anymore and says that he is tired and would much rather me jerking him off. Sometimes when I even do tht he says that his balls r going crazy and so it hurts to go any further and so we have to stop. This is happening really often now...I don’t know wot to do and I think he may be loosing interest, does anyone have any advice?