I have not hand fed any joeys but I would gradually spread out his day time feedings - say 1/2 hour longer between each feeding for a day or two then add another 1/2 hour between feedings etc.
If you have a gram scale, check his weight every day about the same time - just before a feeding to make sure he continues to gradually gain weight.
Once you begin to see him eating from the same plate as the other gliders you could continue to cut down on the formula feedings as long as he is still gaining weight.
You might also try NOT supplementing in the late afternoon so he will be hungry when they wake up - he will be more likely to follow dad to the dinner plate if he does not have a tummy full of formula.