Translation:What will he have thought about the food?
I feel I've not really answered your question. The simple answer I suppose is that the second sentence is simply ungrammatical. All that means is that it doesn't fit the rules for constructing English sentences. The rule for using "will" to express the future requires the following verb to be in its infinitive form. For example, using the verb "run", you can say "He will run" but not "He will ran" or "He will running". Here the verb we want to use is "have" so we can use "will have" but not "will has". I hope this is helpful.
Excellent question. It is much easier for me to show the first sentence is correct. Here is a link that explains the future perfect in English: http://www.englishpage.com/verbpage/futureperfect.html