I won’t worry so much. I think he is about 5th percentile, has he been dropping percentiles? This is just my opinion but I won’t start solids early. Their stomachs are still immature and it may just add to his troubles (indigestion, gas, etc.).
How much did he weigh at birth? Sweet potatoes have like hardly any fat in it though, unless of course you butter it up. Really, breastmilk or formula should be the sole source of nutrition in 1 yr. What formula is he on? I think 24 oz is a good amount especially if you make it higher in cals. Could it be genetics? Are you and your DH tall/big or small? Maybe you can get him to drink more formula?
Hang in there….don’t worry too much! I know easier said than done.