How to tell to my advisor that I want a co-advisor? I am a 3rd year phd student.
I have a good human relation with my advisor, who generally leaves me a lot of freedom in deciding my work and my next steps.

However he recently started a personal business that is capturing all his time and interest. The consequences are:


He started skipping the scheduled meeting, being available more or less once each three weeks.
He rarely reads what I send him, and if he does, he does it in the minutes just before we meet.


In general I feel that he has lost any interest in my supervision, to which he dedicates just the time of our meetings. 
This in generally reduces the quality of what I produce and of the directions that my research can take.

At this point, I am considering the possibility of propose him to add a co-advisor to my PhD. In fact we are having a good collaboration with another professor from a different department (biology, I am in Cs) who seems very interested to my research; but I have no idea if he might be interested in co-advising me.

Might this be a good solution? And how to propose this to him without even hurt the good relation that we have?
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.