M.H.
I'm a lactation consultant and nursing mom. Milk can smell sour if a mom has more of the enzyme that breaks down fat (lipase) in her milk. There is nothing wrong with your milk and can still feed it if your baby doesn't mind. If he does, try heating the milk to a scald (not boiling) after pumping then cool/freeze as needed. This inactivates the lipase. Hope this helps!
M.