First, are you a student or already working? If you are already working, you can give him a tie, which is worth asking for, because men's clothes are not about quantity but quality; it is also good to customize a belt online, and you can engrave what you want to say or your names.
If you are still a student, you can give him a small romantic gift, such as a love letter made of bamboo slips, or go online to customize a pair of couple water cups, print your photos, or what you want to say to him, which is very warm.
The best thing is that you can make some gifts for him by yourself, you can knit a scarf, he will definitely feel very warm if you make it by yourself. Giving a pair of gloves is also good.
Sagittarius men are easy-going and very likable. If possible, give him some cute little things. But no matter what you give, as long as you put your heart into it, he will definitely feel it. I hope I can help you!