My husband is 100% disabled with PTSD, and went from 70-100. He just got 40% for his back, Will he get back pay for the new claim?

CPL Ange Morgan The VA disability takes the largest claim, say it is for something that is 70%. Then they take the remaining 30%, they take the next item say for 40% by itself, it is 40% of the remaining 30%. And so on until you hit 100% total. Its diminishing returns.

If your spouse was already at 100%, then got another claim filled for an item that's 40% there is nothing else to get. Does he draw compensation for not being able to work or any of the other special VA compensations.

Your disability letter and compensation and when it started should be posted on eBenefits dashboard.

You would have likely received the direct deposit for any back pay before you received the letter on the increase.
If he was at 100% (PTSD), the 40% (Back) wouldn't add any additional monthly compensation.
Any back pay depends on:
Date each claim was filed, if filed on different dates, and whether (and when) a claim for an increase was filed for the PTSD.
Example of when back pay MAY be due:
PTSD and back filed same date. PTSD rated at 70%; back denied. Appealed back. At a later date, a claim was filed for increase for PTSD.
PTSD increase granted to 100%, so back pay of difference from 70% to 100% due to date increase filed.
Later, back (from original claim) granted at 40%.
In THIS example only, the result would be a change in the original rating from 70% to 80% from date of original claim.
The difference between 70% and 80% from date of original claim, to date PTSD increase filed.
Note that 70% + 40% doesn't equal 110%, or 100%.
VA calculates it as follows.
Begin: 100% whole person.
Awarded: 70% (PTSD), leaving Veteran 30% whole.
Awarded: 40%. This is 40% of 30% whole, resulting in (.4 * .3 = 12, round down to 10%) 70% + 10% equals combined rating of 80%.
If they were filed on different dates, it changes; either, or both appealed, it changes.

No. There is no more money he can get.