The gods often don't know stuff that others do, that is a non issue. How does he know about it? Who knows, maybe we'll find out.
He's not teaching Goku UI, he's trying to help him figure it out, having a fresh perspective could be what Goku needs to figure it out and Merus being crazy fast is a perfect sparring partner to try to figure out UI since it would help him counter him.
A good example of Gods not knowing stuff is Buu Arc in DBZ with both Shin and Kibito not realizing how powerful Saiyan's were. Especially in regards to their reaction of them transforming super and the Z-Sword stuff.
The Gods in DB may be powerful, but there have been multiple cases that proven that they aren't all-knowing.