It's a hybrid, definitely has maculosa in the mix, looks most likely to be maculosa x stimsoni. Very difficult to say what proportions how how many generations ago the mixing occurred. Visually it looks more similar to a Spotted Python than a Stimson's, which is pretty typical (maculosa traits show up more heavily in hybrids, I've seen low % maculosa blood Antaresia which could easily pass as pure maculosa).
Don't feel too bad, my first snakes were also Antaresia mudbloods which were sold to me as locality pure by the guy who at the time was the most respected herper in the country! One of them lived to the best part of 20 years and at over 5' long was the second or third longest Antaresia I've ever seen. They can be great pet snakes, although you're probably best off not breeding him.