Using former boss as reference

Is it better to connect with him first

You should always converse with your references before you use them as such.

First, you must ask their permission to use them as a reference.

Second, you want to know if this person will actually give you a good reference or not.

Finally, you don't want to blindside them. If they get a reference check call from a recruiter or hiring company, you don't want them to be surprised and thus not able to give you the kind of reference you would like to get.

Call them, if at all possible. You might even find that they are hiring.