Learn English – Word to describe a person who expresses his or her feelings frequently

OP's brother is exhibiting extrafamilial orientation - normal behaviour for young adults of both sexes as they approach the age at which they will leave the parental "nest".

The term distractible can be used to describe someone who is

subject to distraction

Distract is defined as

to draw the attention of (a person) away from something

However, these terms speak more to attention, rather than enthusiasm.

While not an adjective, you might consider dilettante

a dabbler or a person who cultivates an interest without really committing or learning anything in depth.

SUPPLEMENT:

On further thought, the term fickle may be useful

Characterized by erratic changeableness or instability, especially with regard to affections or attachments; capricious