firstly, i hope your DH is now home safe and well.
Yes there is things he can do, the employer can do etc. However, I can only share what would happen if this was in a health setting and not an educational setting. Although, i have a little knowledge of education (governor in school implementing team teach) so will share that too.
Any employee of ours that has either been injured through physical intervention (PI) or has a condition that is excaberated by PI (short or long term) would automatically be reviewed by occupational health and our PI team. The PI team will complete a health questionnaire and if they come across anything that would be made worse through PI then they will refer to OCC Health. Ultimately a risk assessment is drawn up between the employee, their line manager and the PI team in regards what they can do safely and what they can't. Obviously, resources have to be explored and it might be that (in health) certain shifts may not be achieved i.e. lone working. In some cases where PI is required daily and the employee is at high risk of injury then they are moved to another service. (we're talking 3 to 4 staff working on a 6-8 bed ward if comparing to aclassroom setting)
Has your DH been to Occ Health? Had discussions with the PI trainers?
If we have persistent issue with a specific client where the techniques we are using are causing more harm than good (for all) then we refer the whole scenarios back to the PI team and ask for suggestions. It might just be a team review of a specific technique or it might be introducing a new technique.
Our PI team also receive a copy of every incident requiring PI - they review these and question outcomes etc, offer support, advice if needed. They generally pick up issues before I would.
For the child (and what I have seen in our school - its mainstream) is that the safeguarding team (governor level and county level) would be involved with all situations being reported. risk assessments and management plans that clearly state how staff intervene and what techniques etc they can use. This would also be clearly stated within the individual behaviour plan. Again, we have a recent situation where the trainers are coming back in to do some specific training aimed at one specific child.
Are there RA's in place? Is there an IBP? when were they reviewed, do they need addressing now.
Training - when did DH have an update on his training? if he's using PI regularly than an annual review is necessary.
this might be all waffle but I hope there's a glimmer of something that might be helpful.