You'll never stop total beaking.
Anyway, my two cents from recently weaning a bird that took extra time:
I got Albert at 9.5 weeks. He was still on 3 feedings a day. It took 15 weeks (total) for him to be completely weaned. This included a gradual reduction in formula being fed, (never more than 12% of his total body weight) and constant (and I mean CONSTANT) pushing of new foods, pellets, and treats.
For a long time, the only thing he would eat was millet, but eventually started eating others. His progression went Millet > Carrots > Some Harrison's > Broccoli > Some Zupreem Fruit (stopped feeding it because it made his poop colored) > Nutriberries. I credit Nutriberries as the catalyst. Literally bird crack. I also started grinding Harrison's pellets in a 50/50 mix with his formula, which I think gave him a taste because he started eating a lot more pellets after that.
Why are they weaning him onto seed instead of pellets? Seed is awful for parrots (unsprouted) and then you'll just have your hands full switching him over to pellets later on.
Also force-weaning parrots is a massive no-no. I would not purchase a bird that has been force-weaned, for a myriad of mental issues this can cause in the bird (then again, I was cocky enough to buy an unweaned parrot). Abundance weaning has been the method amongst reputable breeders for years.
The first thing I would do once you take is get a fecalgram done by the vet. It will tell you if he has viral, fungal or bacterial complications related/leading to crop stasis.