Willist is correct, at 5 months old, your B&G is not fully weaned.
When I purchased my GW at 9 months old and after being on pellets, fruit, veg for months, I brought my GW home. For the first couple of days, he ate just fine, then I noticed that his food bowls are full and very little food at the bottom of their cage. This told me something was wrong. His crop was flat. I took my GW to my aviary vet here in MD, they advised me that due to change, a GW and other Macaws can regress backwards and be dependant on warm formula again. I tried the warm formula but he would not take it. I had to take my GW everyday for 1 week to have him tube fed. After $800.00 a week, I called my Breeder and demanded that she take my GW back to wean him again. I drove from MD to VA and 6 hours one way and after 2 weeks with my breeder, my GW was then weaned. I picked him up, brought him home again, this time, no worries. Be very careful and look for the signs of regression. Thanks Joe