Question for breastfeeding with supplementing moms/babies

So I was EBF until he turned three months old and then I started giving him one bottle of formula a day usually at night. Then at his four month appointment he had gone from the 30th percentile to the 7 percentile in weight. (This was on September 20th) so doctor recommended that I give him more formula so now I alternate nursing and formula throughout the day, which usually ends up being about six feedings in total everyday. In the morning when he wakes up I nurse him and at night when he goes to bed he gets formula then he sleeps through the night.

So when I nurse him though, he’ll stay on my breast for like 20+ minutes until I unlatch him and sometimes he’ll take the other side too...

Does this sound like anyone else’s situation? Supplementing with formula but long nursing sessions?

I don’t mind nursing him for that long I just wonder if I’m drying up... But I still feel the let downs and I can tell I have milk still. Idk I’m just wondering if maybe I should stop breastfeeding...

I really don’t want to give up. I wish I had never given him formula to begin with because I feel like it screwed everything up.

Does anyone else’s breastfed baby nurse for a long time still?