I don't know if I have any useful advice- just commiseration. My DS7 stomped off in tears this morning because we're working on a challenging math (long division). He is both a perfectionist and an applied learner- can't tolerate too much "instruction" without breaking down.
I'm not a homeschooler, but I wonder if there are things that are less challenging, but useful for your son at age 8? My DS does timed tests at school (math facts)-easy problems, but he's getting fast, so I can see the benefit. My point is that it could be developmental- he may get better at handling frustration as he ages, and there are still plenty of useful things to do in the meantime. As a homeschooler, you have the luxury of jumping around- either to interesting things that he doesn't need specific knowledge to complete, or easier things like math facts to increase speed.
One tactic that I used at home (controversial probably) is to put a small bowl of raisins, jelly beans, etc. out. Whenever DS finished a particularly difficult section of a problem without a fit, or really anything tedious, he would take a small treat. It helped keep him focused and gave him a little incentive/rule- he a bit obsessive about rules, so this works very well.
Also, very small chunks. Because DS goes to private school, I only require one problem per day (no more than 10 minutes) and he needs to check his own answer (via multiplication) before showing me his work. Even if I were homeschooling, I would break hard or tedious work into small allotments.
Good luck.
