I'm not a solicitor but I'm going to assume that there are other factors to consider because I cannot see how it would be a fair outcome for you to get most of the assets and spousal maintenance even after taking into consideration the child for whom you receive DLA.
One way of looking at this is that either you get most of the assets and he has to pretty much start again from scratch in which case he needs to keep enough of his income to do so, or you have a more equal split of assets but you receive maintenance to meet your needs. Otherwise the outcome could be that he retires with significantly less money than you and in his position I would question why I would bother continuing with the stress of a £90k job when I could do something easier and lower paid and get spousal maintenance varied to zero. You'll effectively expose yourself to this risk if you seek a solution with any remaining ties other than child maintenance which the courts normally have limited jurisdiction over (although a SEN child could be a factor that gives the court more jurisdiction).
Also, no solicitor or mediator can really predict what will happen if things go to court. The court has very wide discretion. So two people agreeing doesn't matter an awful lot to me. I've spoken to four solicitors as the higher earner and been told four different things!
Now, I'm going to look at this as a financial planner rather than legally, as that is my profession. I would suggest your asset split is a fair one but the spousal maintenance isn't worth pursuing (he's probably going to fight you over it in court and that could harm your chances of there being enough assets left after legal fees to house you without a mortgage). Allow me to explain why.
I've assumed you would get the lowest DLAs for your child. On your income of £16k, you would receive around £600 a month in Universal Credit. Your combined monthly income of net salary (£1,200), UC (£600), Child Benefit (£150) and child maintenance (£900) would be £2,850.
His combined monthly income would be net salary (£5,000) minus child maintenance (£900) and minus the higher earner child benefit tax charge (£150) so £3,950. However, he would also have to replenish his pension and pay a mortgage (let's say for argument's sake a total cost of around £1,500 a month). So he would be left with around £2,450 after housing costs and already £400 worse off than you.
If you pursued spousal maintenance, the first £600 would reduce your UC to zero and you would be no better off. At that point he would have £1k a month less than you to live on (and another way to look at it would be £600 a month less to either spend on your children or leave in their inheritance, and I would assume an inheritance is going to be particularly useful for your disabled child).
I find it unlikely on his salary that you would benefit by more than about £200 from net spousal maintenance after losing UC, so around £2.4k a year. It might cost around £30k to pursue it in a court so you would need to receive it for about 13 years to break even. That's assuming he doesn't just take a lower paid job and get it varied, or simply avoids paying it at all and has to regularly be taken back to court.
I would suggest a completely different strategy. You need to make sure he does his fair share of the childcare so that you can take back control and have more time to work. He should have the DCs for at least 2 worknights and 2 morning school runs a week so that you can generate your own income rather than being dependent on spousal maintenance, a rug which could be pulled from under you with very little notice.