Your son and my daughter sound like they could be twins, and your situation is a mirror image of mine. So sorry!! I know how hard it is My first step was to call one of the special ed teachers at school that I trusted and ask her if she could recommend a good OT. She recommended two. I called and told them I suspected my daughter had SPD and would like an evaluation. It was a two-hour appointment and within 10 minutes, she told me she clearly had SPD, and that was before she read the several page questionnaire they have you complete. Very frustrating because she had been in special ed preschool since age three, I kept asking the "professionals" what was wrong, and no one ever had an answer (Grrrr. Don't get me started! )
She had OT weekly for a year. Some improvements here and there. Then one night searching the web for the very reason you state in your post ("at the end of what we can handle"), I found a program called the Feingold Diet. It was very, very difficult to start, but not as difficult as dealing with my daughter. Within two weeks on the diet 100% she stopped having meltdowns, she payed attention more (better, but not super), and I felt like I could at least breath and relax a little more (no longer at "the end").
I also tried a home exercise program I found online called Ten Gems for the Brain. We saw some improvement in focus when doing school work. We also added probiotics to her diet, and they bumped up the focus a little too.
She was still plagued with low attention span, very impulsive, no safety awareness and limited social skills. Again I asked the school "what is it?" Then came the ADD diagnosis. I enrolled her in an alternative therapy program (very expensive, but we took it out of some money she inherited from her grandmother for college because I don't see college in her future the way things are going now). You can look at the program if you're interested at BrainBalanceCenters.com. She has only been in it three weeks, and it's a 12 week program, so I don't know if it will help.
I wish you well. Keep searching. There are a lot of programs out there, but it takes a lot of searching to find them.