Baby gone off milk.

My 18 weeks old baby has lost his appetite. For weeks now feeds have been a struggle. He used to take 5 bottles a day of 7/8oz and now it's a fight it's taking about an hour to take 3-6oz. He used to take his 5 bottles over the day where as now he's taking one during the night also. I don't know if he's maybe needing weaned (19 weeks tomorrow). I'm worried when he starts solids he's going to totally go off milk.