From what I've been reading, the replacement shouldn't stump him. Basically, after taking out parts of the old window, the frame that it came in is still sitting in the space. There are variations after that. He can probably find on line installations instructions for whatever you ordered, so he can read up on it. There are also you tube videos which show many different types of installs. Something like you have should be in one of them. Unless your windows were rotting out, it's a lot better to do replacement instead of removing the entire orig frame which not only is a lot more work but a screw up could affect structural integrity. He's just feeling anxious because it's new to him and he's afraid he won't get it right.

Oh, you asked about flush. Interior stops and sometimes outside stops come out, but I haven't seen the replacement seated flush with the interior wall. Maybe some do. Look at what you have now, where the tracks are. The stop goes back on top and sides, same place.

I would seriously advise you to find a professional installer. Your boyfriend is not up for this job if he is stumped by a replacement window. Perhaps he was just trying to impress you and his mouth got ahead of his skills.

Why do I feel like Dr. Laura?

The window should always go to the inside framing, the trim should cover the window perfectly on the inside. Windows 101 the most valuable things are on the inside of your home...

Wow, dig up a 10 year old post for that tidbit......................