What Is the Name of This Book? question

hey, I've been trying to find a book for a few years that I read in middle school and was hoping someone here might be able to help please.
It's about a teen boy living with his parents and wheelchair-bound grandfather, who finds a portal in his basement by accidentally breaking some clay pots under the stairs, which leads to another realm where there is magic and dragons. He trains to become a knight and fights in a battle against an evil guy who wants to take over the kingdom. he eventually falls in love with a girl his age I think named Gwen, and he becomes a beloved hero and is taken back to his world without the chance to say goodbye I think.
I remember the mention of 7 fountains which he then paints during class in the second book and is recognised by a classmate who knows of the other realm and has been training all her life in case she got the chance to go there.
in the third and final book, another one of his friends is taken into the other realm but is greeted by the evil guy and is manipulated into fighting with him, eventually, he rejoins his friend and they all ride dragons into the final battle.
thanks for reading this and I hope there is enough information to go off. thanks in advance for any replies and suggestions!