The basic rate depends on the budget assigned to the advertising. It should increase for each exposed media (TV, internet, print-ad), each country airing the commercial and each year on air. Other factor to consider is the time shooting the gig. Your son should expect a minimum payment per hour and a guaranteed minimum income.
A big issue affecting your final payment is the agency that got you the case. Some of them take a huge commission, seizing foreigners' inexperience, thus you end with short-money for a big case. Some agencies may ask you to sign exclusive contract trying to eliminate competence and reduce your negotiation power, what affects your future payments. You shouldn't give up your legitimate rights without a proper compensation (cash).
You can read more about model agencies for babies in this forum:
Modeling agency for babies/toddlershttp://www.forumosa.com/taiwan/viewtopi ... t=modeling
Note: If you want a quotation PM some details.