My son's eating habits are changing should I be worried?

My son is 7 months old, he will be turning 8 months on the 14th of December. Over the past couple of weeks I've noticed he hasn't been finishing his bottle of 6 oz. He would spit it out around 4oz, and I would try to feed him some solids which he would eat with no problem. Today he has only had 2 bottles one in the earlier morning and one right before bedtime, a total of 12 oz. He had baby cereal of 4 oz and about 3 hours later he was hungry again but did not want his bottle so I fed him gerber which was 4 oz. I know babies still need thier nutrition from formula, so I am worried that he's not getting enough with what he is eating...what should I do? Please any tips or has one else had this problem?