You might try a consultation with an ear, nose, and throat specialist. My little brother was a huge drooler, and still needed a drool bib (or got half his shirt sopping wet) at the age of 3. It turned out that he had hugely enlarged tonsils and adenoids, and I gather that enlarged tonsils can be a common culprit in big droolers. The other red flag in my brother's case was his snoring, and as he got older, my mom started noticing that he had a few incidents of sleep apnea. Another common issue in kids with enlarged tonsils is difficulty swallowing -- my brother would always complain about eating meat, and insist that my mother cut it in miniscule pieces, and one of the first things the ENT doc asked my mom was if my brother had trouble swallowing meat -- the enlarged tonsils made it very difficult for him.