I would not worry too much about the shopping centers in Ocho Rios. The clerks in the legitimate stores are only slightly more aggressive than here in the States, and they will leave you alone if you say that you are "just browsing." The taxi drivers will offer their services as you walk from store to store on outdoor sidewalk, but they move on to the next person if you politely say, "No, thank you." If your heart is on the land tour, go for it -- Prospect Plantation is fascinating!
As to age limit, don't push it. They might not question your son's age, but there's no refund if they throw him off the excursion because he's under the minimum age. If your son is under the age limit, it's better to come back next year. There are plenty of other great tours on Grand Cayman. Check out the "Sea and See" tour, for example!
You might want to catch the Mexican Cultural Show in San Miguel de Cozumel. It's one of the best that I have seen anywhere in Mexico. It's only an hour, so you will have plenty of time to head over to Carlos 'n' Charlie's afterward.