The wretched fish smell (pls help)

My boy Harley is about to be 3 years old in Feb. Ever since i’ve had him at 3 months, hes had these periods where this overwhelming raw fish odor comes off his bum (then he starts licking it)

I know about the impacted glands, but the weird thing is sometimes he’ll go month+ stretches of no incidents, then he’ll have a month (like this one) where hes had 2-3 episodes of it.

It goes away pretty quick. Is this a diet thing? I feed Purina Pro Plan, or is this a thing i just have to put up with and pay the vet to unimpact them multiple times a month?

Thanks