I would think that a little bit of cow's milk would be fine.

With my first son I had to return to work part time when he was about 10 months old, I also didn't have the supply to express and he was in daycare for 2 days a week. My MCNH said he would be fine as he goes over 8 hours at night between feeds. (at that age he was having a morning and night feed). He didn't like straight cow's milk so he had it watered down.

I would think for the amount that he would be drinking it would be fine, if it was going to be on a regular basis you may need to look at forumula but his diet would be varied and contain other sources of dairy in it now anyway.