I see others are encouraging you to stop solids... but, if you do continue with solids, you may want to speak with the ped about rice cereal. We had to take my daughter off of it because it constipated her so badly. She bled while having a BM, and the addition of rice cereal was the only change in diet she had undergone. Anyhow... if you decide to keep providing solids, perhaps discuss the possibility of changing him over to a non-rice cereal with the ped.
You may want to check the can of formula for expiration info. I think most brands advise to discard remaining formula after can has been open for a month. Perhaps there is no real risk and they're just trying to get you to purchase more, but there could be a health reason for this as well.... I would check the can.