The best way to start would be to watch a postseason game on TV with your grandson and talk about baseball cards during the game. Tell him what you love about the game and what baseball cards meant to you as a youth, how they connected you to the game.
Realize that things have changed drastically since the days of one Topps set a year. Realize that your collecting can go in many different ways. And most importantly, realize you can't collect everything these days. Those days are gone. It's too expensive and frustrating to think you can collect everything. The happiest collectors I know focus their collections around a specific and narrow goal. It can be a team, player, set, year, brand, rookie cards, HOFers, autographs, etc. Find out what is important to your grandson if your main goal is to leave the collection to him. Try to narrow the focus as much as you can to make it special to you and your grandson. Get him involved in what you ultimately decide to collect. A narrow focus will help you prioritize your spending and keep you from having to store 1000's of cards you don't really care about. Oh, and realize that buying the cards you really want online is much more cost effective than busting boxes, although opening the occasional box together could be a very rewarding experience.
Good luck. Stay within a budget. Collect some things that bring back your childhood memories and pass those stories onto your grandson. Collect some things from today's game that can build your grandson's own memories and love for the game. Whatever you decide, you can't go wrong if you do it together.