이봐, 네가 그를 얼마나 행복하게 만들었는지 봐 Hey, look how happy you made him. The sentence is a complex sentence. 네가 그를 얼마나 행복하게 만들었는지 is main object 봐 is a main verb. (네가 그를 얼마나 행복하게 만들었는지(를)) + (봐) 봐 is imperative conjugation of 보다 네가 그를 얼마나 행복하게 만들었는지 is a object of 보다 네가 그를 얼마나 행복하게 만들었는지 << 얼마나 + 네가 그를 행복하게 만들었(다) + 는지 얼마나 : how 는지 : a connective ending used to indicate an ambiguous reason or judgment about the following statement. 네가 그를 행복하게 만들었다(You made him happy) is a subordinate clause. 너 is an subject of a subordinate clause. 그 is an object of a subordinate clause. 가 is an subject particle. 를 is an object particle

그를 always means "him". 그 is the pronoun corresponding to "he", and since Korean has marker particles, it can take the form of 그는, 그가 (he - subject), 그를 (him - object), 그의 (his - possessive), 그에게 (to him), 그와 (with him), and many others. (그녀 is the feminine version (그녀가, 그녀를, 그녀의, 그녀에게, 그녀와, ...), and 그들 the plural one (그들이, 그들을, 그들의, 그들에게, 그들과, ...)). The sentence is grammatically correct. It is telling the other person to see how happy he or she made him.