I tend to distrust most auto mechanics. I dont think a mechanic has ever said "Yup, all it needs is a little freon."
If he claims your compressor is leaking, ask for proof. There is a dye they can put in your ac system, and it will show where any leaks may be occuring.
Ive had the ac go out on 2 of my previous vehicles. At the shop, both times they reccomended that the compressor be replaced. I felt like they were feeding me a line, so I told them to just fill it up with freon. The result? Ice cold air coming out my vents. And it diddnt leak out.
Just a thought..