Answering questions about homeworks

I receive many questions from colleagues about homeworks in a certain course which I took a while back. I try to answer them and help them solve homeworks which are very similar to the ones I did when I was taking the course. There is no TA for that course, so they come to me with their questions.

I am not sure if what I am doing is ethical. The professor doesn't know that I am doing this. Should I stop and tell him about this and see what he says?