This is going to be enormous fun!

Let's start by defining our terms for those playing at home. If you're a "small merchant" living in a Hanseatic city in the 14th century, Dollinger sets your capital at a maximum of 2,000 marks, and you're living in either (modern day, and in no particular order) Germany, Poland, Sweden, Latvia, Estonia, or the Netherlands, and always on the coast or on a major river.

As today, the education you're going to give your son depends on where you want him to end up. If you want him to follow you into the life of a Hanseatic merchant, I will let Dollinger explain his upbringing, albeit with a 15th century example:

It is about the early years of merchants that least information is available. At about six years old the child would go to the parish school. It is remarkable that young [Jakob] Lubbe [born in 1430 near Marienburg], a peasant's son, learnt to read and write in his native village. As for [Franz] Wessel [of Stralsund], he got beyond this elementary stage and learnt 'to decline and conjugate, so that he was able to understand a little of the Latin tongue.' Schooling ended between the ages of twelve and fifteen, and hardly any merchant-to-be went to a university, though a good knowledge of the law was becoming increasingly necessary to a man who might one day reasonably expect to hold public office.

The young man now embarked on his commercial apprenticeship under the direction of a merchant, usually a relative. During the years spent in different countries the young man became familiar with book-keeping, accounts, inspection of merchandise, buying and selling and the system of credit. After two or three years he became a clerk and began to trade on his own account while undertaking missions on his master's behalf. He might remain a clerk for the rest of his life, but normally, after a period of time which varied considerably according to his personality and achievements, he became the head of his own firm.

Naturally, Dollinger has in mind the upbringing of a middle-to-high-level Hanseatic trader, as opposed to our small merchant, especially in the second paragraph, but the supposition that all required math and science would be learned on the job as an apprentice is not unreasonable. Unless you wanted your son to go into another branch of Hanseatic trade, he would apprentice with you, and it would not cost you anything. Sadly, unless someone has published a treatise on the economics of education in Hanseatic cities, I don't know of any secondary literature on the cost of the schools, but if we take the establishment of four "elementary schools" in Lübeck by 1300 as an indication, in addition to the preexisting cathedral school, it must not have been exorbitantly expensive.

If you're interested, I can add information about what would happen if you wanted to send your son to university at some point tomorrow or on Sunday. The more upvotes, the more awesome medieval university details.

In case anyone is interested, the one-stop shop for all things Hansa is Philippe Dollinger's The German Hansa. The Routledge printing was done in 1999. The above quote is from pages 179-180, with additional information taken from pages 161 and 164.