OP, I really feel for you. I have an ND child who doesn't have many friends. The children who will play with her don't want to spend time with her out of school, because she's very needy and gets upset easily. It's really hard because DD notices and gets visibly upset about being excluded.
But I get it. DD isn't volatile, but she's very rigid at times. She is also developmentally and socially behind her peers, and has a huge amount of accomodations in her EHCP, some of which probably look like favouritism to a 10 year old because they don't understand how much DD struggles to cope. They just see the fidget toy or the movement break. As much as it makes me sad, I can understand why her peers don't want to hang out with her.
Firstly I have to stress - All behaviour is communication. If your son can be volatile and lash out, this will be because he's struggling with an unmet need. You need to try and work out what need is unmet. I don't know what your son has in terms of support, or what services you've accessed, but this is a list of some of the things that have helped DD.
-The 5 point scale is a really good, visual tool to support ND children in recognising which zone of emotion they're in. In DDs specific we support her in recognising her trigger and start re-regulation activities when she recognises she's at "yellow" (anxious/stressed/upset). It took a lot of consistency, but with support she can now recognise her zone and act on it.
-Social stories are a great way to help ND children understand various scenarios as they break them down and explain them in a way ND children understand.
-Visuals such as visual time tables are very helpful in supporting transitions across the day, because it means children can physically see what the routine is for the day. A visual now and next board can also help hugely with transitions, because it shows clearly what they should be doing now and what they will be doing next. They can always refer back to it which creates a sense of security.
-Brain or movement breaks to give a child the chance to remove excess energy and re-regulate before going back into class. DD and another child with similar needs have scheduled movement breaks with a member of staff in the schools inclusion shed.
-ND children also often experience proprioceptive dysfunction. proprioception is the sense that tells the body where it is in space. It has a significant role in self- regulation, co-ordination, posture, body awareness, focus and speech. We get this input via our skin, muscles and joints. Lots of activities done in school such as writing and focussing need a huge amount of proprioceptive input.
To help DS improve his proprioception, "Heavy work" activities with such as pushing, pulling, wheelbarrow walking etc are helpful. Activities that give deep pressure to the muscle and joints such as tight hugs, massage and lycra body socks are also helpful.
If your son doesn't have an EHCP I would recommend applying for an EHCP needs assessment to get the ball rolling. An EHCP is legally binding and will help make sure support is consistent for as long as your DS is in education.
I've added some social stories and an example of the 5 point scales, I hope these can be of some help to you and your DS
You're more than welcome to message me if you need some further support. SEND parenting can be incredibly isolating.