So with a young, healthy male, the cause or ERECTILE DYSFUNCTION is almost always a mental issue.... NOT mental illness mind you, but usually something like anxiety or guilt or depression.
HERE IS A GOOD SUMMARY ON THE PROBLEM FOR YOU.
Your boyfriend almost certainly does not have any of the medical causes listed at his age so pay particular attention to the section entitled 'Psychological Causes of Erectile Dysfunction'.
This may be a simple fix, a good heart to heart between you and your partner. It may require that he see his physician or a counselor so he can talk this out.
In summary, extremely low likelihood that there is anything physically wrong with him. Almost certainly this issue is a manifestation of something in his life causing stress or anxiety. Your relationship, money issues, job issues, anything that might be causing stress and anxiety. Sometimes this requires a visit with a physician or a few visits with a counselor.
I am standing by if you have further questions for me.