When a worker files for retirement benefits, the worker's spouse may be eligible for a benefit based on the worker's earnings.
Now when he retires and files for his benefits, you can determine what your benefit would be under Spousal Benefits and compare it to what you are then getting. That is several years away even if he files for early retirement at age 62 and receives a reduced benefit.
"Deemed Filing" will not affect your filing for Spousal Benefits when your husband retires and files for his benefits because you turned age 62 before January 2, 2016. However, at the time your husband files for his benefits, his benefit will be scrutinized as to which of his benefits are greater (deemed filing) - his own or as a spouse based on your benefit. Most likely his own benefit will be higher - then you can determine where your best benefit will be greater -
When your husband decides to file for his SS benefits, perhaps both of you should go to the local SS office and have them run all the numbers for you including a benefit for your disabled child IF they meet the criteria for a disabled Adult childs benefit and depending on what the numbers show is better for them and you - SSI / an Adult Child's SS benefit / Family Benefit.
Yea, it is complicated especially since your situation is kinda outside the "norm" - revisit your decision when your husband is ready to file for SS.
I came back to edit and add that the age of "70" isn't any benefit to you in your situation - the age of 70 is only beneficial to those who delay filing for their own benefits until the age of 70 so that their delayed retirement credits (about 8% per year from full retirement age to age 70) are added into their retirement benefit.
Even if your husband waited until 70 to retire, your spousal benefit would not be computed to include these delayed retirement credits. However, these delayed retirement credits are included in any Survivors benefits.