If he talks about all the positives of his past relationship instead of the negatives, then there are chances that he still loves his ex. If he seems to not hold any resentment and bitterness about his past relationship, then its a sign that he is not pretty much ready to have a new relationship with you.

Does my boyfriend still think about his ex?

Its perfectly fine for your partner to think about their ex, talk about them, and even have a friendly relationship with them. But it can help to know the difference between someone whos thinking about their ex in a platonic way, and someone whos thinking about them because they still have feelings.