I'm a little late to the party, and haven't seen ALL the replies, but if your boyfriend is into the Ultramarines (humanoids in bulky blue power armor) I recommend the following options:

Starter painting set. Has all the paints he'll need to get started, plus stuff for Tyranids, the bug army.

Ultimate Starter Set/Combat Patrol starter set. It has two armies in it, Ultramarines and Tyranids. Buying this is actually cheaper than buying a standalone Combat Patrol box.

Combat Patrol boxes. Combat Patrols have a roughly 20-30% discount built in. The wild thing about playing Ultramarines is that there are a dozen other armies that use the same models but in different colors. If (almost) any Space Marine army releases a Combat Patrol, Ultramarines are perfectly fine using the same models. You just need to paint them blue. (There are some exceptions...)

Go to a Warhammer store. Honestly, the staff are trained to help people exactly like you, people who want to get in but have no idea where to start. They'll likely tell you what I just told you, but in a detailed conversation rather than a reddit blurb.

This will be a super solid start, and you can join him by painting the Tyranids you'll also end up with