In my view, there is a very simple argument you can deploy to justify the claim that in He himself the head is he, not himself (assuming he himself is a single NP/DP constituent, which is a possible analysis, but not necessarily the right or only one, since himself need not appear next to he): if the contrary were true, the whole NP/DP would inherit the grammatical properties of himself, rather than those of he, would not carry nominative Case, and therefore could not be a legitimate subject of a finite clause like ...he himself could enjoy the corn. Furthermore, if himself were the head, he would have to be an optional 'adjunct' (note that it cannot be a complement, since himself is saturated), and, since optional adjuncts, by definition, need not be present, himself could by himself act as a DP/NP in subject position. Apart from the case issue that would raise, it would follow that an anaphor need not have a binding antecedent c-commanding it in the smallest clause that contains the reflexive, which, of course, violates Principle A of Binding Theory.
As you can see, there are at least two rather solid arguments in support of the standard view that only he can be the head of the construction he himself (if it really is a 'construction' - a constituent, say at surface level - and has to be analysed as a single NP/DP). In case, however, you should remain skeptical and ask "How do you know that himself is not nominative?", the answer would be straightforward: if it were, * Himself could enjoy the corn should be a mere violation of Principle A of Binding Theory and Him could enjoy the corn should be a possible English sentence, but, of course, it is not. Finally, if you were to ask me, "How, come, can himself carry accusative case at all if there is neither a preposition nor a verb governing it whether it follows he or appears clause finally?", I can contemplate two possible answers: a), perhaps the most plausible one, that there is a hidden preposition (by, or maybe for?) (cf. He did it himself < He did it by himself), as there arguably is in cases like I saw her Saturday/last weekend, or b) that accusative is the 'elsewhere', or 'default', case (since nominative and genitive are assigned only in very specific contexts, subject of a finite verb or specifier of a Determiner or a noun, depending on your preferences concerning NP structure, respectively), an approach that the existence of 'absolute accusatives' in Latin and other languages could arguably endorse.
