We sort of started around that age. If he is telling you when he has gone in his diaper then I think that he is showing signs of being ready. I'd suggest just start introducing him to the potty. Buy him his own potty chair and demonstrate for him and encourage him to use his and tell him this is where you..essentially do your business. My husband and I have demonstrated using the toliet and we just started putting them on the adult toliet (i wouldnt do that if you want him to just use the potty though, my kiddos rarely go in their potty chair these days, and im pretty sure its because we started them on the big potty first lol) The most important thing to remember though is to always stay calm. If he doesn't go on the toliet but shortly thereafter soils his diaper, don't get upset with him. Just firmly but gently tell him that he needs to do that in his potty. Praise him when he does succeed but when he doesn't go just say..."well thats ok, we can always try again later" or whatever feels comfortable to you.
I always tell my husband that potty training is a huge step for little ones that have never worried about their bodily functions before now.
I agree with Jacinda though, don't push too hard. I think a casual approach is always best.
What works with our son (currently 19 months old) when he wants to take his diaper off is a few things...we used duct tape for awhile but he figured that out eventually :) we also will put a pair of soft pj-like pants and then fasten his onesie OVER them...(we did this because my son like to reach into his diaper, after removing his pants, and pull out the contents. makes a huge mess) or if you are really worried about this, we bought a small lock and locked the zipper to the tab of cloth that covers the zipper...or put the pjs on backwards.
good luck with potty training..it sure is an exciting time