1. How can I send the product to him directly from China WITHOUT importing it first to Poland?
Easily. You get a local Spanish agent to clear the goods for you.
2. I assume I must pay duty and VAT in Spain when importing the product under my company. Do I have to pay additional VAT when re-selling the product to the buyer?
If you want to import it yourself and then send it on, then yes, you'll have to do it. Yes, you'll have to pay something additional to the Spanish tax authorities.
In all seriousness, if you only have one client in Spain, it would be best to simply let your client deal with the import formalities.