OK.
Firstly lets answer the last question. There is no longer the concept of custody, only residence.
Since you have no court order in place, you have in effect a residence agreement in which you are the primary carer.
The father has parental responsibility since your son was born after December 2003 and is named on the birth certificate. But don't sweat it. It means naff all in practice as any court would give him it as a matter of course.
The important thing from your point of view is that you've had the status quo for 5 years and for all intents and purposes the father is a stranger to DS.
As for changing name thing, its going to be a fight to get it done without his permission. The only chance you have is to wait a while, see a solicitor and try to convince a court you've done everything you can to find the guy to ask for permission but haven't been able to do so. They may let you do it then but its still not guaranteed. And yes it will be costly.
As for who DS would go to in the event of your death. You can name a guardian in your will for DS. As the father yes he can challenge that. And rightly so. But that does not mean the court will automatically award him residence just because he turned up.
They'd listen to his argument, want to know where the hell he's been and why he's only now turned up. At the same time they'd look into DS, who you nominated as guardian and make a decision as to whom they felt it was in the best interests for DS to live with. One of the factors of course would be the fact the dad is effectively a stranger.
That's assuming of course he even bothers. A lot of men would not want the responsibility of raising a child singlehanded. Gets in the way of shagging and drinking you see.
Hope that helps. Just remember, the best bet is simply not to die!