To truly be able to answer your question, we need to know more. What kind of coins they are, what year, are they proof coins, are they graded, etc...? The mint sells a bunch of different ones. I hope that my answer below does not muddy the waters but if you have any questions, please post them and I will check back in order to answer any questions that I can.
Gold and silver coins have value when compared to the dollar. In general, silver and especially gold coins are a hedge against inflation. So, if the federal reserve prints a bunch of money or if the politicians give us huge deficits with reckless spending, the value of the dollar will decline and your son's coins will be worth more.
Additionally, the dollar has no intrinsic value because there is a theoretically unlimited supply of dollars. All the government has to do is print them from a printing press or create them electronically. However, it is not so easy to just create more gold or silver because there is a limited supply of these metals. So, gold and silver will always be worth something.
Lately, meaning since about 2002, the value of the dollar has been in relative decline. The purchasing power of the dollar has fallen greatly. However, gold and silver have risen quite dramatically. So, some of the coins that your son got a few years ago have probably increased in value.
My personal recommendation is to hold the coins that your son has, buy more gold and silver and wait. When the government gets its spending under control and when the federal reserve no longer sets interest rates at very low levels, then that would be the time to consider selling the gold and silver.