@Viktoria Vasile thank you

How much water or energy drink does he drink during training? and how long before swimming does he eat and how much? I'm not an expert but the only thing i can think of or makes sense to me is maybe his breathing during training, maybe he should try to really force all the air out of his nose before he takes a breath (while he is swimming) and when he is resting at the wall try to take deep and slower breaths ( i know its difficult during a hard set) but HOPEFULLY it will help.

When i went to Italy for junior worlds open water I only tapered 3 days before the race and i asked my coach why we are tapering that late he said because distance swimmers need to keep the distance longer as they could loose maybe a bit of fitness if they taper 3 weeks before. sprinters can start to taper 3 weeks before because the don't usually swim more than a 100m which means they don't need that much fitness and they just need to work on short burst of speed (like 25m and 50m) and they will still be fit enough to do a 100m. But i should say that the open water was 5km. So for a 1500m i would only taper 5-6 days before the meet. Maybe even 7 days depends how you are feeling and i would still keep the distance up but not as much as usual.

I am a breaststroker myself, and I would also struggle with knee pain. So then I asked my coach ( who specialized in breaststroke himself) to look at my breaststroke kick and he said I'm not kicking right so he changed my kick and never struggled with knee pain again, (also swam faster as a bonus). So know i swim up to maybe 2.5km of breaststroke a session (longest I've done was 4km of breast) and never complained about my knee. So i would ask your coach (if you haven't already) that knows breaststroke well to look at your stroke and maybe you could change a few things and that could solve the problem (Hope this helps) Also it could be a flexibility problem As Lewis Dawson said.