how do i improve the temperature of my sons room?
What is your heating method? Most are somewhat adjustable, though this may not be obvious if you don't know what to look for (i.e. baseboard hot water generally has a flap that can be open, partially closed, or fully closed on the top section of the baseboard, which affects heat delivery.)

If you turn up the heat delivery in the room that is colder, and/or close it down in the room that is warmer, the room temperatures should become closer.

Not sure where you are going with the plastic - 6 mil plastic is not an effective insulator. If you have attic access and can't afford to "double the insulation" go buy what you can afford (6 mil plastic costs money - spend that money on insulation instead) and put it over his room, if that is feasible. If money is really tight you may be able to get help with insulation from a heating assistance program or by asking for leftover insulation (most people who do an insulation project end up with some) on craigslist.

Another alternative would be to swap rooms (so you take the colder one if you are concerned about your son being too cold) and throw on some extra blankets.

Edit: since you have forced air heat, partially close the vent or vents supplying the warmer room with the thermostat. If there are not built-in louvers to do this, place a book or board over part of the vent, or tape something over part of it if it's a wall vent. This should reduce the rate of heat delivery to that room, so the system wll run a little longer to heat it up to the setpoint on the thermostat, and by doing that, the other room will become warmer relative to the room with the thermostat. You could also have a HVAC professional adjust this deeper in the duct system - but in general we as end-users lacking certain tools and information the HVAC pro has should not start adjusting dampers (in the ducts) willy-nilly (it's fine to open any louvers (on the vent) in the son's room and close them in your room, or to block vents in your room - "opening up" a system too much can burn out the main fan motor (this is a counterintuitive but important point - blocking of vents reduces the load on the central fan - opening them up too much can overload it.)

In general, what you are describing is a system with poorly balanced heat delivery, which a good HVAC technician should be a be able to correct. But you can do a lot of it yourself by closing down or partially blocking vents in all the "too warm" rooms until the "too cold" rooms are warmer.