Cancun

Cancun is probably the safest place in all of Mexico for tourists. The place has no reason to exist except as a tourist destination. As a result, the authorities make sure that tourists are treated well and enjoy a safe environment. My wife and I visited there about 10 years ago. We rented a little Suzuki jeep and drove all over the place, seeing the Maya ruins in Chichen Itza and Tuloum and the tourist attractions in Shelha and Shkaret (not sure of spellings). We had a blast and felt a hell of a lot safer than we would in similar places in the USA. And, btw, neither one of us is exactly fluent in Spanish; that didn't present a problem. I hope you and your son have a terrific time!

I also have been to Cancun. It is reasonably safe especially if you stay within the resort areas - which you actually never have to leave. The most dangerous thing in that area is the ocean - the surf can be very rough so be especially careful. The resort area is so built up that except for the beautiful beaches and tropical feel and look you would think you were in the US - every typical restaurant in the US is there - Dunkin Donuts, McDonalds, Hard Rock, Outback, etc.

I would also suggest if you do want to visit the ruins, with a small child, I would take a tour rather than drive myself. Although it is reasonably safe, if you get lost or anything else, it would be much better to be with a tour group. Also, many resorts in the area may have kids club, although I would think that 2.5 would be a bit young for those.