At many of the big box stores, there are sold large spikes that are used in the construction of railroad tie retaining walls. They are about the diameter of your little finger and nearly a foot and a half long.

Because you have a fiberglass roof, a three pound hand sledge will easily drive these spikes through your ceiling and your roof. Make sure that the extra length on top is bent backwards, so as not to create an additional parasitic drag. These will hold your ceiling in place.

If that isn't to your liking, pick up some PL polyurethane construction adhesive at the same stores. There are several types; one "professional" varient features a fast and strong initial tack. The glue is spread onto the insulation, the insulation is pressed into place and then removed. Given a couple of minutes, the glue becomes very tacky to the touch. When none will transfer to your finger, reinstall the insulation, and it should stay in place until the glue reaches its maximum strength, which generally occurs overnight.

As was stated, errant pieces can be blocked into place until everything is secure.

hang on there I', not sure that will work.

is he supposed to glue the foam to the fiberglass ceiling then glue the beadboard to the foam?

No. The glue is for attaching the foam insulation to the ceiling. The beadboard will have to be supported with a fabricated ceiling structure that includes walls and "rafters", (ceiling joists) This is especially important if the consideration of adding an A/C unit is contemplated.

The side walls will support the new roof sub structure. If the ceiling components are made of hardwood instead of the lumber yard common Spruce or Pine, it will be of sufficient strength to support the hundred pounds an A/C unit weighs.

The beadboard can then be affixed to the new structure, with the rigid insulation filling the in between gaps.