I am going to unhook a 200 amp service for a double wide home so it can be moved out. homeowneer wants a temp. panel put on service post so contractor has power to work with while building new house. I am going to unhook at the 200 amp disconnect on service post. My question is can I run a piece of #2 ser from the disconnect into a 100 amp panel to provide temp. power, or should i put a 200 amp panel outside on post then move it into house when done.

If you can stay in the tap rules you can and that needs a raceway in most cases but when you are "outside" it says "suitably protected from physical damage".

fuse it at 100A, Protect it and you should be fine. Ground rods too.

I am guessing this is an existing 3R 200a disco on a post, typical for manufactured homes and there should be a ground electrode system installed. I assume he wants to use one of those 100a temp power boxes that has a main breaker so this will just be a feeder tap situation. The only real issue is protection of the tap conductors and separating neutral and ground in the temp power panel. They are usually set up as a service (from my experience). He did say "SER" so I am guessing he knows that.
It sounds like a 240.21(B)(5) to me.