I think that there is a serious problem here. Serious in that you want it fixed and the 'mechanic" wants more of your money for not doing a good job in the first place when he replaced your radiator.
But I don't know a lot of facts so I might be wrong.
A few questions.
Why was the radiator replaced?
Is this a radiator shop where you had it replaced? What does the local Better business Bureau think of this place and/or do they have any past complaints about them?
Does the coolant come out after car is stopped and engine turned off?
Have you had someone else look at car (friend, relative, another shop)?
When the radiator was replaced, did the guy get all the air out of the system? (this involves opening a air bleed port in cooling system while filling and running engine (at the right time and sequence). If not done right, it can cause problems.
I suspect that possibly there is a bad hose or a clamp that was not installed correctly or it could be as bad as a leaking new radiator or even a faulty or wrong radiator cap installed (this happened to one of my friends. When he had a new hose installed, the guy put some other cars cap on by mistake).
You need to watch the temp. guage constantly until you get this repaired. Also, have you had your oil level checked lately? I check mine at least once a week and change my oil and filter every 3 thousand miles. But then I'm kinda anal about things.
Here is some light reading on the subject.
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Papa Ray