I think he already told you to go outfit the truck, so there's no problem with you going and outfitting the truck.    If you wish to make an offer to buy the stuff out when or if you leave, I see no problem with that.  It might make you more apt to fully load it without 'feeling bad' about spending too much money on stuff that you need but perhaps can do without, and that would probably be better for him from a productivity standpoint.  I also no problem with him either accepting or declining your offer.  Assuming that he is going to continue in business after you might retire, he will still need the tools and truck, so he might prefer to keep them. 

If he's going to provide you with tools, that means either he trusts you to take care of them and treat them as your own  (ie, don't lose them, basically) or else it means he doesn't think you will carry the tools to do the job if you have to buy them yourself.  If he's sending you to set yourself up on his bill with a 'blank check' so to speak, I'd say it's safe to assume we're talking about the former rather then the latter.