Car wont start dont know if it will go into gear

Why does he think it is the flywheel, does the truck still turn over and not start? If it does not turn over than could be a few things, not getting voltage to starter, a bad starter solonoid , bad wiring connection, to mention a few..

I just got a new starter and installed it and its doing the same thing it was doing nothings turning over and I put a new battery in not long ago

You have a good starter and good battery, When he turns to start the truck does the starter turn the engine over? With a good battery and starter the tell tale sign of the flywheel being bad is that the starter would click. I have a firebird here that does the same thing. replaced the started and altinator as the battery has a slow drain. What I would do is check all the wires coming from the battery, even though they might look good there may be some corrosion built up so it does not make a secure circuit to the starter. Also found out that a bad ground on the body would do the same thing. Trying to evaluate the system before you go into the expense of pulling out the transmission.