It seems that your boyfriend is not only mature and stable, but also a relatively careless man. In fact, there is no definition of romance. As long as there is love in your heart, even having a meal or taking a walk together is a kind of romance. Giving a gift on his birthday can also be romantic.
Men are careless, so they need women's thoughtfulness and delicacy. Then give him a thoughtful gift. This depends on your usual attention to him. For example, his wallet has been used for a long time and is old, so give him a new wallet. Or give some other men's products, such as ties, shirts, leather shoes, men's care products, etc., which are all good choices. Mature men generally don't like things that are too fancy, so you should choose good quality and taste.
Or give some commemorative gifts, so that he will think of you when he sees the gift. For example, cartoon ceramic statues, made by comparing real photos, or handmade things, personally embroider a pair of cross-stitch pillows for him, or make a love album, which will definitely make your careless boyfriend feel your delicate love.
You can also give handmade chocolates, the kind with DIY greetings.
I hope these suggestions can help you.