I'll buy and wear the same clothes as him.

1. I will buy and wear the same clothes as his. OK, but a little unnatural. 2. I will buy and wear the same clothes as he does. This is the best and clearest sentence. 3. I will buy and wear the same clothes as what he is wearing. Not quite right. It would be fine if you deleted the 'what'. 4. I will buy and wear the same clothes as him. Language purists might say that this isn't correct, strictly speaking, but many native speakers do use this form. 'Him' is an object pronoun, and the object of this sentence is the clothes, not the person. To be correct grammatically, the last word should be 'he', but this sounds awkward to the modern ear. That's why the safest option is to use sentence #2.

I think 2 is correct.