I finished weaning all of mine, I got them when they were down to 2 hand feedings of formula a day. If he doesn't eat soon - they can get malnourished quickly, you might want to go back to formula for a week or so, and taper off again. I would want him eating seed as well as millet. Not much nutritional value in millet!
You can get the formula at any pet store, and just mix it with warm water until it is a gravy consistency. I have hand fed formula using a syringe, but my favorite method is taking a plastic spoon and lightly heating the spoon on both sides with a lighter, molding it into somewhat of a funnel. The birds eat out of the end of it and you can just tap the spoon to get the formula to move forward so they can get to it easily.
Mine were completely weaned at 7 weeks, but I got them all at 4 and 5 weeks, so got work the process out myself.
I could also chop up broccoli very fine (as tiny as seed) at that point and they loved it.
Hope this helps somewhat!