Arm gets tired jacking him off

In a new relationship, he is m26 I am f21. He struggles to orgasm, but stays hard for hours. Normal sessions last like 2-3 hours, I want to get him off but after like 30 mins of penetration I’m done so we go to handjobs or blowjobs. My arm gets tired after again around 30 mins, he likes me to go hard and fast but it makes my arm sore… what do I do? I feel bad I can’t keep going. The last guy I was with lasted 2 minutes so I am not used to this lmao, it was also a couple years ago. Is it normal?