The stool sounds normal.
As long as the stool is not hard pieces, your son is not constipating. My son also strains a lot when he was young. But it gets better along the way when he learns how to pass motion.
I was also on 80% BF that time. I noticed that when I changed his milk powder from Similac to NAN, he is poo-ing better. But different babies react to different formulas. So some of my friends whose babies are on Similac, are ok... Just that mine's not.