My boyfriend has no idea I've been secretly topping up his perfume for two years

My boyfriend (25M) and I (22F) have been together for a while and have lived together for the past two years. We didn't grow up with the same financial circumstances. His family had to be careful with money, while mine was fortunate enough to be comfortable. That difference still shows sometimes, especially when it comes to spending money on himself. A few years ago, we attended a gathering with some of my old school friends. He noticed someone's perfume and couldn't stop talking about how nice it smelled. He considered buying it, but the price put him off. I eventually bought him the same fragrance as a gift. He was genuinely happy with it, and it quickly became his everyday scent. Later, I offered to replace it whenever he finished a bottle, and he agreed. What he doesn't know is that I've been doing something slightly different. I noticed he was trying to make the perfume last as long as possible, so I bought a bigger bottle and started secretly topping up his regular one whenever he wasn't around. It's become a little routine over the past two years. He thinks he's just getting through his perfume normally, but the bottle somehow never seems to run out. I earn enough to afford it without worrying, but he still struggles to justify spending money on himself. I started doing this because I wanted him to enjoy something he loved without feeling guilty about the cost. Now the secret has gone on for so long that admitting it would probably be more awkward than keeping it. Somehow, a simple little gesture has turned into a two-year secret.