My bf doesnt get hard even when he is horny. some days ago during the stimulation he could get a bit hard like 17% and he orgasmed. what is the cause?

See endocrinologist : does not sound like psychological. See PCP OR endocrinologist to look for physical causes like hormones etc.

Here are some...: Very likely, what he experienced is at the time of being tired or over-exerted or / and trying to have coitus numerous times during a short period like 3-8 hours. If such, he would do just well for the first or second time, but start not being able to keep up in his 3rd or 4th... attempt. Thereon, the harder he tries, the worse he may be in achieving erection due to fear for failure kicking in...