Is it possible that he’s doing it to get a reaction out of you? If it is the way to deal with that is to ignore it (as safely as you can) and give LOTS of ott attention for other things. I don’t mean walk off or shrug your shoulders, but for instance you could say “let me show you how to climb down safely... go backwards, one foot, then another, that’s it! Great boy! And down, down, down we go...you did it. All the way down (big squeezy hug) so you haven’t reacted to him climbing but you’ve given a huge reaction to the process of coming down.
And then try and give him big, positive attention before he climbs, and if he still races up, take him down without saying a word. And then resume the positive fun and attention for another activity. (This is exhausting, I won’t lie, but if you can outlast him, it’s very effective)
On the other hand there may be a sensory seeking element in his need to climb. (You might have an idea which it is, but if you don’t just work on the assumption that it’s both)
Try some activities that give really strong proprioceptive feedback - crawling, rolling, being squashed (carefully) under cushions, hammocks etc. You may have to make these really fun games to compete with the climbing. At 18 months babies have developed a great sense of mischief and fun, so play on that. Get him to crawl under a chair and act really confused when he comes out the other end...then make the tunnel longer by joining chairs together. Get him to complete an obstacle course. Make a sausage roll (roll him up in a blanket) and pretend to eat him as he kicks himself free.
It’s exhausting being with a toddler so you have to plan activities that wear them out more than you. Scavenger hunts in the garden can be brilliant (you sit and relax and send him off to find random items).
Alternate a couple of high energy activities with a calming sensory activity (water/sand/play dough/finger painting). There’s a lot to be said, by the way, for doing these sorts of things in the bath or shower.
Google strings like occupational therapy games at home, for ideas you can adapt that don’t need you to order special equipment.
And one more tip, it can be safer to teach him to tell you that he is going to climb (and eventually to ask) and do it with you, than to try and forbid it outright. That’s a perfectly fine way to deal with it too if it suits you better.