I helped my ex husband file for social security disability benefits around 2014 for his illness and he was approved benefits. I was unaware at the time of doing his on line application of this time should maybe count as credits for his 4 years served in the 1990's . My ex husband has passed in 2016 from his illness and I am single mother raising his son that is 15, I wanted to know how do I go about making sure that his record reflects his credit if he was entitled to receive this on his social security record.
I assume that you're referring to military service credits. Military wages have been subject to Social Security taxes since 1957, so those earnings are posted to a veteran's Social Security earnings history just like wages from any other employer. Social Security also credits deemed military wages (DMW), which are extra wage credits that are added to the veteran's actual military wages for periods of active duty during the years 1957 through 2001 (https://www.ssa.gov/OP_Home/handbook/handbook.09/handbook-0953.html).
Starting with 1968, DMWs were automatically added to the actual military wages for active duty veterans, so no action is needed to receive the proper credits. Therefore, if your ex-husband served in the military during the 1990s, all of the military credits to which he's entitled should already be posted to his Social Security earnings history. And, all of those credits would have been included when calculating his Social Security disability (SSDI) benefit rate, as well as the benefit rate payable to eligible survivors.
You can check with Social Security to see if their records show that DMWs were included when calculating your son's benefit rate, but I really don't think it's necessary. I worked for Social Security for 36 years, and I'm not aware of any case in which DMWs for years after 1967 weren't properly credited.
By the way, you don't mention whether or not you are receiving benefits in addition to your son, but since your son is under age 16 it sounds like you could probably qualify for divorced mother's benefits if you aren't remarried (https://www.ssa.gov/OP_Home/handbook/handbook.04/handbook-0416.html). However, your benefits could be subject to full or partial withholding if you're working and earning too much (https://www.ssa.gov/benefits/retirement/planner/whileworking.html).