Just ask him to hold your hand it's that simple. You really don't need to ask to hold his hand if you do then it might be a sign to get a new one :S and you can do a trick and hold his hand unless you actually want him to hold Your hand
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He probably thinks that you are not comfortable yet. Just slip your hand into his' while you two are walking or sitting together . BUT DON'T LOOK AT HIM WHILE YOU'RE DOING SO! Or else he would be uncomfortable and think you're desperate. And don't worry if he doesn't want to, I mean, he wouldn't force you into doing something you'd regret. Make sure you're hands aren't sweaty either, no guy wants to hold sweaty hands. Be confident or else it would lead to an awkward situation.