If I read your post correctly... it sounds like you "mentioned" to your contractor that you wanted Jeld-Wen when you were originally "negotiating" the contract, and he told you they prefer to install LBL. In other words neither Jen-Wen nor Milgard were documented anywhere in your contract.
Did you at least list R-Values and window function similar to the Jeld-Wen in the contract?
Without specifying the exact manufacturer in the contract you have no recourse. If LBL was listed in the contract then hire a lawyer. If your contract didn't specify things like vinyl versus aluminum, or they must have argon gas, or listed a minimum R-value etc., then you more or less left the entire decision on window selection to your contractor.
If this is the case, I'm afraid you have little recourse. Rule of thumb, (for future reference) if a contractor cannot install or build something specific to your request... then find a different contractor!
I had a customer who insisted I install Pella windows. The customer even specified the series and model number. The windows were very expensive and would increase the total job estimate/cost ("and" would reduce my profit margin on the job!) It's easier for me to add my markup on a less expensive window. That said, I would have at least made sure the insulation qualities on my choice would be similar to the Pellas. In the end, I installed the Pellas because the customer refused to budge. In all honesty I could have installed a different window with the similar specifications and insulation properties for less money to them (and put more money in my pocket)... but they knew what they wanted. If I didn't make the arrangements to install the Pellas they would have found another contractor who would.
Next time... Two Rules:
1) "Never forget who's holding the checkbook"
2) "Get it in writing"