It is not unusual for someone who is depressed to isolate from others.
The best thing is to continue to encourage him to socialize and to do things with you whether it is going for a walk, out for a coffee, anything that will get him out of the house and engaged in doing something with you. It may not be a long activity but just something that you know he used to enjoy doing with you.
You could also offer to go with him to see a psychiatrist for medication and if he wants to go to counseling. The medication will be helpful but he would also need to address some of the issues that are bothering him such as workholism in order to avoid other things or to compensate for something that he feels he should.