Обалдеть is, literally, to become a blockhead, an idiot. It's a colloquialism. 1) It can be used as an exclamation: - "Обалдеть!" - "That's crazy!", often meaning "crazy awesome", or just something unexpected. It is actually a shortened form of: - "Обалдеть можно!" - literally "You can becume dumb (from this)", "This can make you dumb", meaning that teh thing we are talking about is something unexpected. 2) It can be used in phrases in a similar meaning: - Стас просто обалдеет, когда узнает! The speaker expects Stas to be very pleasantly surprised with the news. 3) When we say that someone "обалдел" in past tense, we often mean that someone has become extremely brazen, to the point of losing common sense. - Представляешь, он без спроса взял деньги, отложенные на отпуск, и купил себе электрогитару! - Совсем обалдел!

It's correct to say " он обалдеть какой хороший/добрый/крутой" - it means " I admire his kindness/coolness". So this is correct in case You wonder some person or some thing and tell another person about this guy or thing. in positive context. "он обалдел " means that He wonder somebody or something. It can be both positive and negative context.

Обалдеть/обалдевать (Infinitive) It means be shocked, surprised, grow insolent, grow impudent, grow stupid, stunned, spacey, spacy Примеры: Когда я тебя увидел, я обалдел, ты такая красивая сегодня (быть в восхищении) Ты обалдел? Ты зачем ее обидел? Теперь иди и попроси прощение (переступить черту дозволенного, обнаглеть, быть дерзким) Иногда и от стакана с пивом можно обалдеть (одуреть от стакана пива) Я обалдеваю с него ( я в шоке от него)

No, but you can say "Он просто обалдел" - it does not necessarily refer to distant past, it can also mean that he has "обалдел" a few seconds ago and is still in this state now.

So I can say он просто обалдеть?