Been wondering this for a while

How would you add 'Do you know' on to the start of a sentence in Japanese? It just sounds better than simply saying 'Where is he?'
Could I say something like 'Do you know where he is?''

"Do you know" part would come at the end of the "where is he" phrase in Japanes.

"Kare wa doko ni imasu ka?" (Where is he? - with polite speech, with "masu" form)
"Kare wa doko ni iru (no)?" (Where is he? - in informal speech, with "(r)u" form of the verb)

"Kare ga doko ni iru ka wakarimasu ka?" (Do you know where he is?)

"...wakarimasu ka?" is the "Do you know..." part. You will need to know the dictionary form (or "ru" form) of verbs to make a sub-clause out of the "where is he?" part.

in addition to wakaru, you can use shiru (slightly different nuance .. use it if you ''care'' less about the answer, I take it)

…か知ってる？
...kashitteru?

Q:「あいつどこにいるか知ってる？」
A:「あいつなんて知らん！」

forgot to ad .. huge Japanese mistake is "do you know .." instead of "have you heard of .." and the like. I'm sure you've come across this if you've ever spoken to Japanese people

they fsck it up because they are directly translating how you say it in Japanese

"do you know Japanese anime?"

sounds stupid right? in real English you'd probably say "are you familiar with" or "have you heard of" but in Japanese, you say something like アニメって知ってる？ as with a lot of other things, much simpler than the excessive words required by English in this regard