bf cant “keep it up” during sex

My bf and I are doing a long distance relationship and hes staying with me for the summertime. For the first couple of days the sex has been great but lately he cant seem to stay hard during sex. After a minute or two he goes soft during sex. He says it isn’t because of me but I cant help to think that way :/ Has this ever happened to anybody? and what do you usually do in a situation like this? I dont want to make it awkward for him or make him feel bad about it but im not sure what to do