It's stressful on them, to force feed, but it is necessary if they stop eating altogether.
Is he drinking on his own, at all?
If he is just NOT eating (but still drinking) it is best to hand feed a tablespoon of seeds, for each meal. They don't appreciate it, but it's less stressful for the grown up birds. Then allow him to drink.
There are other methods of feeding them in the resource section, you can check them, I use the less invasive method, though it's messy.
IF the bird isn't drinking, you need to soak the tablespoon of seed for 10 minutes, then drain and hand feed. (never re-use soaked seed for later) Open beak and gently put some seeds over the tongue to the back of the throat and allow the bird to close its beak and swallow. Make sure the bird is kept warm, first and foremost.
What are the symptoms the bird is displaying now, and what does the poop look like? Are you gving it supplemental calcium, and vitamins, and any digetive aids??