At my school, we use the following strategies:
PLP (Personal Learning Plan) for students regardless of placement on SEN register - this has helpful info for teachers including the students' likes/dislikes (in general and in class) Students have been involved in writing the PLPs and typically they say things like "I don't like it if you tell me off in front of the class, so if you need to talk to me, please do it outside the room" and "I like working by myself, so if you ask me to do group work I will find it hard" This is really helpful when planning.
Timeout cards - if students know that they might react violently/aggressively to situations, they have a card which they can show, and then have 2 minutes outside the room to calm down.
A named and trusted person (in our case, a TA - he's young and a trained counsellor) they can go to if they need to - at any point during the day.
A school policy called Discipline with Dignity (not ours - google it) which is exactly what it sounds like, and means that staff are strongly encouraged to deal with behaviour in a specific way, and that if we don't, we are pulled up on it.
It is also policy to use "language of choice" with students - ie "Tom, if you choose not to sit down/carry on with your work/have a go at question 6, you are choosing a yellow card/to stay in at break time with me to complete it/whatever appropriate sanction" and to use our praise system 4x as often as the sanction system (simple green/yellow/red on board) SLT will pop in on learning walks and it's one of the things they want to see - 4x as many names on the green list as the yellow or red list. Name on the green list = house points.
Of these, the PLPs are most useful. The timeout cards are always issued in consultation with parents and are obviously reviewed if it is felt that the student is abusing it.
Might be worth a conversation with the SENCO/Inclusion person at the secondary school so systems can be put in place to support. If your DS was at our school, his PLP would have something like "XXX responds far better to encouragement than to sternness. If XXX loses his temper, then coaxing helps"