can my son collect under his father whose on SSD?

Yes, this may be possible.

Your child can get benefits if he or she is your biological child, adopted child or dependent stepchild. (In some cases, your child also could be eligible for benefits on his or her grandparents' earnings.)

To get benefits, a child must have:

• A parent(s) who is disabled or retired and entitled to Social Security benefits; or
• A parent who died after having worked long enough in a job where he or she paid Social Security taxes.

The child also must be:

• Unmarried;
• Younger than age 18;
• 18-19 years old and a full-time student (no higher than grade 12); or
• 18 or older and disabled. (The disability must have started before age 22.)

http://www.ssa.gov/pubs/10085.html#a0=0

A wrinkle in your case, however, is that the ex-husband has no obligation to provide child support from his SSD. However, in most cases, the child benefit described above is in addition to any benefit the parent may receive. I am not sure how the SSA handles such a case. The following is the only information I could find on that :

https://secure.ssa.gov/poms.nsf/lnx/1507210001