Poor little guy. It can be rough to miss mom and dad. This is fairly common and sometimes children get to feeling like they'll never see them again although they can verbalize it as it's more of a feeling or subconscious thought.
You did all the right things. Your love is and will carry him through, what a good auntie you are. Just keep showing your love and when he brings it up just tell him he'll see them soon. Maybe he can make a calendar to count and mark off the days. Maybe he can make them a home-coming gift, something that would take time and be a project to last for a few days. Maybe something like sewing together some shape cut from felt and filled with lavendar or made into a bean bag so they can all play with it, or a wallet or little purse or something they could pin to their shirt that says I love you. Just an idea.
Maybe he can draw some pictures about his stay with you, the things you're all doing. Maybe he can write a letter to them. He could draw pictures with a short sentence or two about a story you've read or a chapter book.
Just keep letting him know you love him and understand and reassuring him.
The best to the sweet little guy.