Hello, I am Rafael. Thanks for asking your question - I'm here to support you. (Information posted here is not private or confidential but public).
I am very sorry to know about your situation.
For sure something must has happened for him to have changed this way towards this girl and now isolating this much.
he has the right to keep his privacy for sure but it should not imply stop taking good care of himself and comply with core rules and responsibilities personal and family ones.
When a teenager gets depressed, he could isolate, not be open to talk, socialize and more. His feelings must be respected and family show caring and empathy as you have been doing, but it should never mean passivity allowing so enabling withdrawal and lack of accountability and his need to develop a healthy life even more when he's feeling down.
Staying too much time in his room and spending too much time on the computer are both very dysfunctional and unacceptable behaviors. Allowing them would enable further dysfunction fueling depression and any other mental health problem a person may be facing due to personal-life issues. So please be very clear your role needs to be very assertive and truly supportive. If he does not want to take about his personal emotional concerns, that's OK but it should not justify isolating and not doing what he needs to do at home, school with family, etc depending on his circumstances.
Does it make sense?
Children do need unconditional love, understanding, support and gentleness too, which should not be confused with any form of codependent enabling of unhealthy behaviors, which would only worsen any problem the child may be experiencing.