I would say : "All he could do was stare at the wall" You could also add "to" before stare but it sounds a bit more natural to say the sentence without "to" You need "do" after "could" in your expression because modals are always used with main verbs, except when answering a question briefly in the affirmative or negative e.g. "Yes I can" I can't explain why you can say "stare" and "to stare" and why the first sounds more natural. One day I hope I will be able to explain this!

I explained it in a similar question here: http://www.italki.com/question/269723 Or, you could search for each phrase in quotation marks.