Hello. My name is Marc. I'm a licensed attorney and I will be happy to assist you with this problem. I can certainly understand how anxious you feel about this egregiously unfair treatment from your landlord. The short answer is the landlord can legally deny leases to those with criminal convictions - whether misdemeanors or felonies. There is no MO or federal law that specifically prohibits landlords from denying leases on the basis of criminal history.
But, that's not the end of it. It could be that the landlord is discriminating against you and/or your friend and is simply using his conviction as a pretext to deny him. While it is generally legal to discriminate on the basis of a criminal history, it is illegal under MO and federal housing laws for a landlord to discriminate based on age, sexual orientation, race, disability, etc...
In your case, I wonder what else the landlord is trying to accomplish by denying your friend. Perhaps he wants you to move out as well for any of the reasons above? As you observed yourself, it makes no business sense for the landlord to deny your friend, given how you've been a loyal tenant in good standing and are prepared to pay increased rent for your friend.
When considering whether to rent to someone with a criminal history, a landlord should consider how long ago the crime occurred, any extenuating circumstances surrounding the arrest or conviction, whether the crime has any relevance towards the tenant’s ability to be a good renter, and any evidence of rehabilitation.
In your case, the crime was not serious and your friend's probation is almost complete. Also, there is no indication that the rent won't be paid. Therefore, the misdemeanor should not suffice as a reason for denial.
So what do you do now? First of all, there should be nothing preventing him from staying as a guest for the time being. Even if he's not on the lease, you are still entitled to have guests stay with you, after all. Next, ask the landlord if your friend can provide any written references - as to character, employment, honesty, etc. Some landlords will take this into consideration. Keep in mind that the landlord might simply want to "cover his ass" against the possibility of any future lawsuits. For instance, if your friend wound up assaulting another resident, the resident could sue the landlord for negligence, based on having accepted a violent criminal into the property. Clearly, this is not the case with your friend, however. So, see if you can allay your landlord's concerns in this regard with letters of reference or anything else (perhaps even a letter from his probation officer).
Another less desirable route would be to sue the landlord based upon the above-discussed discrimination. The landlord would have the burden of showing that it treats all prospective tenants with the same harsh rule - not just your friend. Before doing so, however, consider writing a strongly-worded letter putting the landlord on notice that you believe you and your friend are being discriminated against and that you don't believe denial of his tenancy has anything to do with his misdemeanor. Note: This should be your second approach AFTER you've tried the above-discussed offer of references.
I hope this will resolve quickly and favorably for you. And I hope you're able to enjoy the holiday. Again - I discern no legal basis for him to literally be removed and banned from the property tomorrow.
I hope my answer has provided you with more insight as to your issues and options. If so, please be sure to rate my answer, since that is the only way I can receive credit.