School Safeguarding Lead here.
OP, first of all I am so sorry to hear this happened to your son. You have had some sensible advice on this thread but, as with any thread on the topic of safeguarding, there is some well meaning but somewhat misguided and inaccurate stuff as well.
Don't contact the LADO, their remit is to manage allegations against adults who work with children so this is not relevant to them. Also don't contact Ofsted, they don't even inspect Independent schools. Even if it was a state school, it's not necessarily true that a child on child assault would trigger an inspection as some have claimed on here. You need to follow the schools formal complaints process and then if you're not satisfied with the response, escalate to the body who inspects Independent schools.
As for the 4 day suspension, I agree this is not an appropriate consequence but it could just be a starting point. They may be looking to expell the two boys, but expulsion requires the school to collate evidence, investigate and put all the necessary paperwork together, so it is normal to suspend in the first instance to give them time to do this. You would be entirely reasonable to ask for clarification around this though.
You would certainly be within your rights to report this to the Police, both the perpetrators are well over the age of criminal responsibility and what your son has described is potentially a common assault. Lots of people on this thread are shocked that the school haven't involved the Police, but it's actually not usual practice for schools to report child on child assaults that occur on school premises to the Police directly (except in cases of serious sexual assault/rape or where a weapon is brought into school) as it is generally up to the victim/their parent to decide whether they want to report. However, the school should have discussed this with you and personally, if I was handling this incident as DSL, I would certainly be encouraging you to report. The likelihood is that the Police will say they are happy for the school to follow their disciplinary process and take no further action, they are generally reluctant to criminalise young people for incidents like this IME. But that doesn't mean it shouldnt be reported. For all you know, these young men may even have previous involvement with the Police.
As for the "council safeguarding team", if people are referring to Children's Services (commonly known as social services) then I think there is some misunderstanding as to their role. Schools don't report incidents to Children's Services, they refer children. CS would not accept a referral for a child based on a one off incident of assault taking place at school, unless there were additional risks such as abuse/neglect at home or exploitation in the community. Their position would be that school need to follow their internal behaviour and anti-bullying policies (you should be able to find these on the school website) and conduct their own risk assessment to safeguard the children involved. There would be no role for a Social Worker unless there were additional safeguarding concerns relating to any of the children involved.
In your shoes I would want detailed information around supervision and monitoring that happens in the boarding house, such as who is responsible for supervising children in their free time, what are the rules around children entering each other's bedrooms/dormitories and how are these enforced, how often children are checked when they are in their rooms, what arrangements are in place for them to get help from an adult etc and, importantly, whether they are planning to make any changes as a result of this incident. Essentially, you need assurances that if your DS were to return there would be robust supervision, monitoring and reporting arrangements in place to prevent something like this happening again.
I would also expect the school to offer your DS some form of emotional wellbeing support, possibly counselling, due to what he has experienced. If this hasn't been mentioned that would suggest they haven't fully the emotional and psychological impact of this incident on him, which would concern me as a parent.
Personally, in your shoes I don't think I would be happy for my DS to return if the perpetrators were returning.