You have been left in an unfortunate position, but unfortunately one that comes up far too frequently.
When customers are doing the planning things can go badly wrong.
Builders do tend to go ahead and deal with any difficulties as they see fit. Communication is not a strong point.
If you are to cut the holes then explain that this is an extra time requirement and will cost x amount more.
You should really give a written quote / estimate so that you do not get "oh I did not realise x was not part of the cost you quoted"
Remember to try and stay calm and explain clearly, and in writing if necessary, at all times (once you can do this tell me how!)
You should be able to reach your cables as you have left plenty there, unless he has cut them short to make it easier to get the ceiling up!.
Then using a metal / stud detector for finding the plasterboard nails or just tapping for hollow spaces and dead joists should allow you to plan where the fittings should go. Then you can start (gently cutting the first hole, just the drill into the plasterboard as short as possible to avoid catching unknown cables and once the hole is started just use the cutter, cut out the last paper with a knife, unless you can see the hole is clear through the drill hole.
Once you have a hole cut check it is where you expected (ulp) and if so then you can cut the other holes in relation and fish out cables with a wire coat hanger or if you are posh with a cable rod with a hook. If the cable is not visible or reachable by hand (if you do not have ham hands like me) then a flexi camera can look for them to aim the coat hanger, or a small mirror on a rod for the cheaper option.
Good luck and stick with it, bad news travels fastest, if you can remain professional then you should get a good response after the job, with any luck.
I had one very bad job that was driving me crazy and I thought I was burning boats at a fantastic rate and later they recommenced me to some one else!
You can never tell.
Sympathise but do not empathise.
