Here are your parts
Replacement parts for FRG FRT18IL6DMJ | AppliancePartsPros.com

Here is the wiring diagram
ftp://ftp.electrolux-na.com/Pr...F/Anderson/240379018.pdf

he thinks its either the defrost timer or the temp control switch

From your description I do not think it is either

defrost timer - remove the rear covers at the back of the freezer so you can see the evaporator coils. If they are not heavily iced/frosted up you do not have a defrost problem.

temp control - unless the condenser/compressor fan is shutting off the compressor is getting voltage and it is trying to cool down to set point.

Sounds more like low freon. Check the evaporator coils if they only get cold/frosty where the freon enters the coil, it is probably low freon

Here is a good link
http://www.applianceaid.com/refrigerator.html

Check out #20 and the example pictures part way down the page

what is approx cost for appliance tech to add freon?
Sorry cannot help you with a cost.
Perhaps call a couple local repair shops for a quote.
If you post your location maybe one of the techs who posts on this site could give an estimate.

Does this mean it's leaking freon ( 2 year old fridge)
Yes you have a leak.
There is no use refilling it unless the leak is found and fixed first.