You absolutely need to tell him. Do not delay. Since you have limited time and you will be apart for awhile, why not buy a parting gift that he can remember you by? It doesn't have to be big or expensive, just memorable, and preferably something he'll keep with him and remind him of you (i.e., not something generic). When you've made your selection, contact him, and either ask him to lunch or tell him you need to see him before he goes away. Present your gift there.
Practice what you're going to say. Make sure he has your contact information and a photo. Better yet, take a parting selfie if it feels right. Set aside your fear and just GO FOR IT. People regret opportunities they don't take. If you'd like, you can pour your heart out in a card as well in case you don't nail it verbally. I'm rooting for you.
You've got this.