what's going on with my hubby? possible tmi

My husband works hard during the week and then does farm work after he gets home. Originally I was just contributing it to the fact that he's tired and stressed out but now it's happening more often than not. During sex I'll suck him off because that's his favorite lol but he won't be able to get it back up, like no matter what I do to him he can't get hard again even if he's still horny. For example, last night I sucked him off and we went for round 2 this time he got hard but was unable to finish. what are y'alls thoughts? life in the bedroom has gotten kind of boring too so that may also play into it?... any suggestions to spice it up? I would like to stay anonymous but ill answer any questions with an update! UPDATE: yes he could go for round 2 or even 3 or 4 before and be just fine! It's just now been a issue