Would you forgive your BF if he left without saying good bye? And then returning almost 2 years later...?

sounds like this young man did the only thing he could to help him get through a very devastating time in his life. try talking things out with him and maybe he will be able to explain why it is he did what he did. Talk is free and generally there are no strings attached. Grief can have a very devastating effect on a person and can take many years to overcome.