If I had to bet on it, I'd say a further 1.5 - 2 inches length; with noticeable increase in girth. 3 months is plenty of growing time
There is a lot of factors that come into play if he grows only that much there is something wrong! In my 200 gallon my red devil and Midas it's been 4 months and they're about 4 inches maybe 5. They grow like crazy especially flowerhorn. I had a flowerhorn at 12 inches and grew to about 16 inches in three years. Once they get bigger than 8-9 inches takes a while to grow large.
Water quality and the amount of food will be a huge factor. If he's sized in a small tank he won't outgrow the tank he just won't grow.
When I was a child I had a pacu in a 10 gallon tank wonder why he didn't grow big. My father got me a 55 gallon and he literally grew in a year 5 times from what he was. Housed in a 200 gallon before I sold him he just got too big and they're known for being tank breakers so didn't want to come home to a flooded house.