Firstly, it's important that you let your boyfriend know that he's not alone. If you can't visit him, it might be a good idea to call him, or to let his close family and friends know what he's going through.
You should also advise him to get in touch with the Samaritans, who provide help to those with suicidal thoughts, or emotional stress that they're struggling to deal with. Suicidal thoughts can be very overwhelming, so it's important that your boyfriend knows that he's not alone.
There are a number of places to seek help if you're feeling suicidal. As mentioned earlier, charities like the Samaritans specifically deal with cases of suicidal thoughts.
Friends and family can help you to remember why you want to live, and show you that you're loved dearly by those within your life. If you're don't have many friends or family members, you can join online discussion groups that aim to help people with emotional distress.
If you know that someone is having suicidal thoughts, it's important that you tell the people closest to them about it. Although you may feel like you're betraying your boyfriend's trust, you may just be saving his life.