With your UK licenses, you will both be able to get an exchange, see http://www.drivetest.ca/en/license/ExchangeReciprocal.aspx
It looks like your son with a brand new license might be able to get a G2 license. This is the 2nd probationary license in the Ontario graduated licensing program. The big difference here is that if he gets it, he can drive alone. If he shows up in ON with no drivers license and has to do the Ontario program from scratch, they will start him off with a G1 which means he can only drive with an adult by his side. The G1 period lasts for one year unless he completes an approved drivers course, then it is 8 months but if he arrives with a license and can get G2 right away, it would be better for him and better for you because you would not have to come with him while he drives.