1. He seems to be happy.
2. It seems that he is happy. (formal style)
3. It seems like (as if) he is happy. (informal style)
In sentence 2 and 3, I assume that;
1) the conjunctions - 'that' and 'like', 'as if' in informal style - lead not a subject complement but a real subject clause (that is, impersonal subject + complete intransitive verb + real subject clause).
2) 'seems' is justified to function as an complete intransitive verb, which leads a real subject clause.
Appreciate your clarification in advance,