I messaged a tech recruiter on LinkedIn with a greeting like “Dear Mr. Smith”, and he replied with “Hi Jen”. I used his last name for the second message as well since he never signs off his messages and I wasn’t sure. Would it be weird to greet him using his first name from now on? (he’s fairly young)
Would it be weird to greet him using his first name
No. It is fairly common to greet using "Hi" + "first name", infact it is becoming de-facto mode of greeting now at least in tech world. Though it is alright to use "Dear __", the latter is user mostly for quiet formal communication where you do not know the person well enough. Having said that it is safe to start communication with "Dear" or "Hello" and once you gain some rapport with the person, you may use "Hi".
Some good discussion here.
9 times out of 10, you should use their first name in every communication. This isn't 1980 anymore, so unless you're applying to a very traditional company, skip the formalities. Don't believe me? Look at what actual hiring managers have to say.
< rant > This bothers me so much I wrote a post on how to craft these communications correctly. < / rant >