If I reroof using architectural tiles can I lay over existing roof without removing old tiles.?

You can, as long as they are not curling and there arent too many layers. They won't last quite as long as if a complete tear-off is done, and the warranty may be void on a re-roof. If you live in a region that gets snow, make sure some sort of ice protection is used. Most roofers overlook this on a re-roof, and ice will back up under the new shingles. It is cheaper, but not as good.

Your question says tiles over tiles. I'm hoping you mean tiles over shingles. Or shingles over shingles. Actual tiles over real tiles is not going to work. I can only go by what you said, but I'm fairly certain you mean shingles over shingles. This can work, but frankly it doesn't save enough money to risk it. Be sure to put 30 lb felt over the roof if you do tear off. Contrary to what people think, 15 lb is not waterproof. IF your climate calls for it, use the ice dam protection as others have suggested. Sounds to me like the roofer just wanted to get out of some work, since tear offs are labor intensive, he could give you a few hundred off and still make more money by not having to pay the workers for the day of tearing off.

Many insurance companies and local codes will NOT allow 3 layers, but it is fine to do what you want as long as it is only one layer currently on the roof.

As long as your not talking about clay tile roofing you can have up to 3 layers

As long as the lumber in your roof is sound and it has only one layer of shingles you are going over otherwise the weight of THREE layers of shingles is too heavy.