My two cents: get an evaluation. It could be a tendon thing, it could be a neurological thing, it could be SPD, it could be nothing--a phase he'll grow out of. If he's doing it 65% of the time and he's also had other developmental delays, it's time to just check in with an evaluation to be responsible and safe. It's not about whether or not you like labels or believe in Autism or SPD etc. etc. It's about the fact that your child is doing this 65% of the time and it's not seeming like normal development. Knowledge is power. Get an evaluation. If you get a label you don't like--then disregard it and just take away from it the information or services you need.
"Do I believe these are real conditions? Of course, but have you seen the checklist for sensory processing disorder. Check the link below it was provided for me below by one of the lovely mothers. If you looked at that thing you would see that the criteria fits just about everybody."
My daughter is now receiving OT for Sensory Processing Disorder. I was initially very wary of the label and had the same reaction to the questionnaire when I first read it (these questions are all over the place! everyone must have SPD according to this!). But think of it this way: some people say they need glasses to see things that are far away. Some people say they need glasses to see things up close. Isn't that wacky? Just about anyone could fit somewhere in that spectrum! Maybe that's not the best analogy but do you see what I mean? We're not used to thinking of sensory input that way but why wouldn't it make sense that there would be a broad spectrum of ways that the sensory system could also have problems--just like any other system of your body?
SPD is about the brain misinterpreting sensory input. Sometimes that means they aren't getting enough feedback (high tolerance for pain etc.) sometimes that means heightened sensitivity (can't stand to wear clothes with tags etc.). Those are two distinct reactions--it's not like the questionnaire means you test positive if you check a box here or there in contradictory areas.
If you look at that questionnaire and just check a few boxes, then no, that's not SPD. If you check a lot of boxes, in the same few categories, and these issues are impeding development or enjoyment or participation in normal life, then it could be SPD.
The only way to really know if to have an evaluation by someone who's trained, like an OT or a developmental pediatrician etc. I went to the OT because there were a few areas where it was really bothering my daughter and causing her to not enjoy activities like a kid her age should. But at the evaluation they showed me that she was lagging in some areas of development that I wasn't even aware of. So I'm very thankful they called my attention to it. I don't care about the "label". It's about getting resources to help your child. Who cares what anyone calls it? I don't care if my child has "SPD" (and I don't care if you think it's a crock!) if that's what we need to call it to get her the help she needs--so be it!
If your child is under 3, you can get a free evaluation by Babies Can't Wait. If your child is school aged, I believe he could get a free evaluation by the school district if you ask in writing.
One last note about all the label hysteria: A label does not change one thing about your child! They're still the same kid. What a label CAN do is get your services and resources to HELP your child that you couldn't otherwise get. I have two relatives who have been diagnosed with Asperger's Syndrome (high functioning Autism). I thought my relatives were out of their mind when I heard this--these kids are smart, sociable, funny--how can they have Autism? But guess what? They're still those same great kids with or without that label. And with that label they now get individualized education plans that help them thrive in their classrooms to overcome problems that weren't readily apparent to me but were preventing them from reaching their full potential in the classroom.
I would just caution you to step back and ask yourself why you're so caught up in labeling or not labeling. Who cares what anyone thinks? It's just about gaining insight and knowledge and care for your child. If someone with training and knowledge spends time with your child and explains their thinking and why their experiences lends them to lean one way or another, then you at least have the information to make an informed decision or an informed opinion. Looking at one questionnaire online is not the way to form a judgement on whether or not SPD is "a crock". I fear I'm rambling so I'm going to sign off...
I wish you the best for your child.