A couple of things here stand out to me.
First, in general, crib time outs aren't a good idea. They are convenient, but they also create negative associations with the place of sleep, which causes problems later on; around this time children often go through a phase where they don't like to go to bed (as they're starting to be aware of the ability not to!) and it can be extremely difficult to get them asleep as it is, not to mention if they associate their bedroom and crib with time outs. A neutral space is better, if possible.
Second, time outs in general aren't highly recommended until three. This article recommends using them, but not having them be solitary; instead, have a minute or two of calm down time with mommy. We found teaching our son deep breathing helped some - he would repeat "calm" in a breathy voice and it would often calm him down. Two year olds (or almost 2 year olds, as yours is) aren't ready to follow rules yet, so traditional 'stay over there until the timer goes off' won't work.
Third, get a regular bed - whether that means converting your crib to a toddler bed using a short half rail, or buying a fun race car bed, or just a small bed that's lower to the ground - now. If he can climb out when he's mad, he can also do so when he wakes up in the middle of the night, and he very well could hurt himself. 21 months is somewhat late in my opinion to be doing this for an average to above average sized child - for my two boys, both of whom are 85th-90th percentile, we converted at 18 months.
Finally, something that every parent has to learn at this age is how to spot and avert temper tantrums when you can (you'll learn how, I promise) and help him learn to stop them himself. This helps reduce the frequency, and where you can, teach him skills (like deep breathing, using his words, etc.) to control his frustration - and at this age it is mostly frustration, not anger. Time outs are often not helpful for these, because he doesn't understand why he can't have a chocolate, or why you don't understand which milk glass he wants, or why he has to go to bed. If the time out isn't calming, then it's ineffective - it just frustrates him a more, likely. Focus on using a calm voice and talking to him about why he needs do or not do something, wait out the tantrum, and try to let him develop the skills for handling frustration, rather than trying to stop the tantrum.