Awww. This is so sweet. There is nothing to get worried about, they are just kids.
My son was in love for the first time at age 4 in Pre-K. He came home one day and told me, "mom I like 2 girls, can I have 2 girlfriends?" I say no, you must choose one. But of course I worry a little bit, I thought about myself, and then asked my husband if he felt in love when he was 4, the answer was not... At that time my son and I were having therapy for his behavior for ADHD I had the chance to ask the professional, he told me it was normal.
Later on my son picked up his girlfriend, she was a Japanese girl, that girl knocked my son's heart. You have no idea how in love my son was, he wrote and drew pictures of her and him together holding hands, or he gave flowers to her, all these pictures were over and over everyday on the walls, in his homework, in the books at home, in the patio and he would tell everybody how in love he is. He plans to marry her, after he finishes school of course, he said they are going to have 2 kids, he put names to the kids, he also told me, he will be a surgeon, he will buy a big house for her and their kids.
We were in the military and we used to move every 2 years, so after we moved, he was sad, he wrote a lot of letters to her, he wondered if she missed him, if she was thinking about him, if she was sad because he is in another country, he was a sad boy. I tried to comfort him, and told him that yes, she is missing him a lot, but the same way we move, she may move soon also, but communication is important. The real thing is that that girl didn't know my son was so in love with her, and time means healing for all of us.
Later on in kindergarden he got another girlfriend, then in first grade. In second grade he and his best friend got girlfriends also, but again, the girl didn't know they had boyfriends hahaha. Now he is in 3rd grade, and he didn't mention anything about a girl yet. I asked him, he says no, and smiles.
All kids are, and feel different, my second child is 5 and he didn't have a girlfriend yet. Wish you the best luck for you family, just enjoy your son's feelings, and let him know how in love you are with his mom.