Can another player help him out, like a tutor or a guide between turns? maybe make him a quick guide to combat actions.
We are going to need more detail than just your OP. As it is we are just guessing as to what the problem is.
What sort of questions was the player asking? Are they basic rules questions (e.g. how do I disarm someone), more obscure rules questions (can I 5ft step out of difficult terrain), requests for GM rulings to enable an action (can I swing on this chandeleer to get across the room) or questions about the scenario that aren't rules based so much as requests for information about the situation at hand (does my character know if a hammer works well against these skeletons)?
What sort of options does the player have in battle? Are they overwhelmed with a whole smorgasbord of potential actions that they have to sift through? Are they frequently in a position where pretty much everything they do is terrible and they waste time trying to figure out some course of action that isn't horrible? Are they pondering their turn despite their choice really being as simple as where to stand next to the monster while they whack it with a sword?
Why couldn't you answer the questions the player was asking? It does kind of come down to what the player was actually asking, but you still should be able to shoot out answers to questions very quickly (unless the answers are very complicated, but that's not really the player's fault unless they don't understand basic rules elements). Basic questions you should just know. With obscure rules issues you can just acknowledge you are unsure on and then pick a way to run it for the moment. Rulings where the rules are silent can be done by making a system of resolving the action on the spot (make a DC20 acrobatics check to swing) or you can just say outright it can't be done. Questions about the world around the PC you again should be able to answer on the spot, unless the questions are ridiculously specific.
What do you mean by "He kept going on about it" when you told him his turn was over. Did he say he figured out what he wanted to do and you told him "nope, too slow, wait till next turn"? Was he being sullen and muttering while you were resolving other players turns. Was he loudly complaining about you being "unfair" for taking his turn away from him. In other words, how severe was his behaviour, exactly?
What was he doing during the turn of others. How did he have the time to do other things when the rest of a full turn cycle takes less than a minute and a half according to you (which I suspect is a major underestimation). Does he completely lose interest in the game the moment he is no longer in the hot seat. Does he lose focus when the turns of others start taking a while? Is he disinterested in the whole combat (and why do you think that is)?
What was he doing during most of his turn. Asking questions? Staring at his spelllist or the table wondering what he could do? Trying to figure out how to roll an attack roll. Coming up to speed with all the changes that had happened since he last pulled out a distracting electronic device?
Is there anything else notable about that player, the rest of the table or the game you run that might explain why this problem is happening?
There. Answer most of those and we should be in a much better position to provide advice. Otherwise we are just trying to read between the lines of your post and guess the problems happening in your game.