My son is self harming.?

The other day I accidentally walked into his room while her was changing. I noticed cuts on his thighs and calves. I pretended that I didn't notice them and walked back out. I gave him acouple days to see if he would come to me but he hasn't said a thing and still acts the same. I haven't noticed any changes in his behavior he doesn't seem depressed and he's still going to school doing what he needs to do at home and every once in a while he'll have an attitude but nothing out of the ordinary. He's 15. My wife (his mother) is in jail for acouple more months and im only 34. I've never dealt with anything like self harm. Im unsure what to do at this point