For an In-Trust Account for a son, who claims the dividends on their income tax return?

The person who put the money into the account claims the interest/dividend

From CRA:

"Generally, when you invest your money in your child's name, you must report the income from those investments. However, if you deposited Canada child benefit payments into a bank account or trust in your child's name, the interest earned on those payments is your child's income."

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/rprtng-ncm/lns101-170/121/menu-eng.html