My son went to University fall 2017. He did not claim tuition fees as I paid them. Can I still claim them if my son did not include this in his return?

Your son needs to declare the tuition amounts on his return, even if it was you that actually paid them.

The CRA requires that the student claim the amounts, using as much of the amount that is needed to reduce his tax payable to zero. Only then can any unused amounts can be transferred to a parent or spouse.

Does anything change if he is already receiving a return?

If he's already getting a tax refund without entering his tuition amounts, then no, the tuition will not change the bottom line of his return. But, he still needs to report it on his return and then indicate how much he's transferring to you. Because unused amounts can be carried forward to a future year, the CRA keeps track of these amounts and tracks them for you.