The best way to know about it is consulting a lawyer. There r many consultants who hv no consultation fee. That’s the best option
Let's simplify this for the original poster as he wrote the son is "21+ years".
If the boy is 22 years of age or older, he cannot be sponsored as dependent. (Also if the boy is not considered a dependent according to IRCC definitions, for which one notable criteria is that if a child is married or common-law, he is no longer a dependent.)
If he is 21 years of age, they will consider him as a dependent as long as the application is submitted before the child's 22nd birthday. (At least that is my understanding)
So if he is 21 and the father wishes to sponsor, the father should submit the application as soon as possible.
All of this is in the application package materials (i.e how to determine the son still qualifies as a dependent).
This does not necessarily require a consultant or lawyer. DO NOT DELAY APPLYING BEFORE 22ND BIRTHDAY IN ORDER TO GET CONSULTATION - that delay could well prove decisive.
Next: if that is not the case, he should look at other PR application procedures (economic class and a few others) to see if any way to qualify, but those may be substantially more difficult. Use of a lawyer/consultant for those other procedures/classes may indeed be more useful or required, up to the father and son to figure it out.