What's really going to reassure you is scheduling a consultation with a speech-language pathologist, but not necessarily because there's something to worry about. It's simply that it'll take the uncertainty off your shoulders, and the professional can give you specific strategies to stimulate his language development at home, whether they're needed or not. The important thing is that you don't have to guess on your own by reading stuff online.
That said, the fact that he understands everything you say to him is a pretty good sign. A lot of kids at that age are in a phase where they understand a ton but say very little, and then between 2 and 3 they just click and start rattling off words nonstop. In the meantime, you can do simple things: read him stories, sing him songs, describe what you're doing day to day. It's nothing complicated, just making natural use of the time you spend together.
The speech-language pathologist consultation will give you peace of mind, and if there happens to be something worth working on a bit more, the professionals know how to do it in a way that feels like play to the kid. You've got nothing to lose by checking it out, and lots of times insurance or public healthcare covers it without any problem, so it's not some crazy expense.