How to tell to my advisor that I want a co-advisor?
A legitimate way of supervising PhD students is to start fairly intensive, but as they progress gradually reduce the amount of supervision/help. The purpose of a PhD thesis is to show that you can do independent research, so the advisor needs to enable that by not being too closely involved. This is obviously a tricky balance act, but that could be an alternative explanation to the behavior you see. The obvious way to find this out, especially if your relationship is good with your advisor, is to just ask her/him. If the problem is that (s)he is easing you to independence, then you know and you may talk on how to fine tune that process. If the problem is that (s)he is too busy with other things, (s)he would not mind the help of a co-advisor.