The Ghana international spent time on loan at Signal Iduna Park in 2009 from Tottenham Hotspur.
When asked about his relationship with Dortmund coach Jurgen Klopp, the 26-year-old conceded that the two are friends who talk "regularly".
"Bundesliga is always interesting to me," he told Eurosport. "However, at the moment, I'm still wearing the red and black jersey of Milan.
"At the moment Klopp is still busy with his pre-season preparations, so he doesn't have much time. We both have a lot of things to do. But we chat regularly and that won't change as he is like a friend of mine."
Boateng made 11 appearances in all competitions during his stint with the Black and Yellows.