I gather that it will be a brief time before he's eligible for the earliest possible enrollment date. My inclination is to get the money as soon as possible unless
there is likelihood of significant income before full retirement age. Social Security in normally not taxed unless you exceed a certain income. Last year that amount was ~ 25,000 which includes 50% of SS benefit plus all other income - earned or not. Also, when earned income exceeds something like 15,000 they reduce SS payments by $1.00 for every $2.00 earned. I could not advise anyone earning above minimum wage to accept the benefit till full retirement age.
Beyond that, I'm not sure I really understand the entire question. Certainly, there is no retroactive benefit.