Yes, the reading part could be, but the combination of struggling to complete school work (getting it sometimes and not getting it other times) + the fact that he seems to get really tired while doing simple things like eating make me wonder if there's some underlying physical cause. Hypotonia, for example, can be treated (it requires a lot of regular, physical exercise). Knowing what's going on with this child might help his mom parent him better. If mom knows there aren't physical/neurological issues, then yes, it's a power struggle. But I've seen too many parents spend a couple of years fighting with their child, finally get a diagnosis and go "Oh, so s/he wasn't just lazy/stubborn/whatever." And those couple of years of assuming it was the child's 'fault' did damage to the relationship, and to the child's self esteem.
Our son has some very mild special needs (sensory). Because of those, he has not been able to learn to swim (he can't bring himself to put his face in the water, the sense of floating freaks him out because his vestibular senses are poor and he can't figure out where his body is or what it's going to do.) My brother has very similar issues, and I distinctly remember how frustrated my parents were with him and how he was often accused of not trying. It wasn't that he didn't want to try, it's that his body went into full panic mode when he was in the water. He couldn't try. Because I know my son can't try (at least right now, as he gets older, it gets better), we avoid that power struggle altogether. I still don't know the best way to teach him to swim, but I know that forcing swimming lessons right now is pointless.
So, before declaring this a power struggle, I'd want to know if indeed there was something going on that makes it hard for this child. But jumping to the conclusion that your child is lazy or that you as a parent are making this a huge power struggle, maybe looking for other causes makes sense.
Personally, I'd wait until after the baby is born, and the family has readjusted to let mom's hormones and son's reaction to a new sibling work themselves out. But if the problems persist, yes, I would recommend a medical evaluation.
I understand how getting a disgnosis could help a mom come to terms and stop negatively labelling the child, but it seems to me to just be replacing one label with another...I guess the point for me is...stop forcing your kids to do stuff they are not interested in. Why do you need a label to not fight with your kids over reading or swimming or eating or whatever? I've never had a productive argument with my child. I have never won an argument with my child. His vision of the world can be summed up with the quote: "I reject your reality and substitute my own."
Instead of getting mad, I try to get creative.
Obviously, if a diagnosis helps you find strategies to do that, then fantastic, but labels can come back to haunt a child, so unless something is really glaringly awful, I'd probably start with diffusing the struggle, and THEN go in for a medical evaluation.