Can I send aspirin to someone who lives in Belgium?

I don't know who told you that they don't have aspirin over the counter in Belgium. It took me all of five minutes to find two online Belgian pharmacies selling aspirin, where, as far as I can see, they don't require a prescription, and I'm sure there are many more. I just clicked on the two first hits I got, using Google.

Keep in mind that not all countries use the word "aspirin" for acetylsalicylic acid. In Denmark e.g. we often use the name "magnyl" instead, and something similar could be the case in Belgium.

As for whether you can send aspirin to Belgium. I would assume so, since it doesn't appear to be a prescription medicine there, but there might be an issue with the amount if you send a lot. Also if you are sending them from outside of the EU, shipping, and any added costs such as VAT, import tax, custom handling fees etc. could end up making them very expensive.

uh yes we do.... aspirin, ibuprofen, paracetamol, etc... all OTC here, same in the rest of the world

Yes!, but why?  Aspirin is available all over Europe. Possibly in different guises, but available. 

Why? Sure they can go out and buy it easily enough.

There is no reason to send aspirin to someone in Belgium. 

you need to ask the post office this question