If it is dyslexia, tread carefully because I believe that is a protected disability under the ADA. Some HR departments are only good for helping you generate documentation to fire people while others are actually able to help improve. In either case, to protect yourself it might be best to engage someone from HR. He may have disclosed that disability to the company, in which case you will absolutely want to make sure that any type of coaching you give him won't be construed as failing to provide a reasonable accommodation. You will also want documentation to be on your side in the event that things go sideways in the future.
Does this person formally report to you or do you share a manager who has just offloaded the responsibilities to you? It's a lot harder if you're just his dotted line and will probably require you to engage not just HR but your manager as well. Of course if he's not your direct report, you have the option of letting him fail on his own. Make sure it's known that you can't do your work and his and as his work starts to have an impact on customers and projects, be sure to refer those complainants to his manager. If you're always fixing his mistakes and working 60 hours a week to do his job on top of yours, then as far as your manager is concerned, there's no problem. When he has customers, sales reps, PMs, etc banging down his door, then he has to do something. Don't get sucked into the trap of "but the customer suffers if I don't stay here until 8pm to do his job after I do my job".
If your oversight of this person is more formal, then your job is to do one of three things:
- Get him to develop professionally and start improving
- Find another role for him where his strengths will be useful and his weaknesses will be contained
- Vote him off the island
You need to sit him down asap and put it in no uncertain terms that his work product is not at the level that you, the company, or the customers expect. Show him his work, show him an example of what you expect, and explain where he is missing. Do not give him an Oreo - that is, don't try to say well you do this great, and I really like this, this sucks butt, but you do this well. Be straight and direct and explain where this train is heading. The work he does is not meeting the standard that is expected of him, customers and PMs want him off accounts, and if he can't produce work for any accounts, then you're going to need to find someone else to do the job.
Next, you need to commit to providing assistance and coaching for him while stressing that you can't do the work for him and that this specific coaching exercise has a timer on it. You're not going to spend the rest of his time with the company as his editor or ghost writer. You're willing to work with him, but he has to make drastic improvement. Give him a structure of what the process is going forward. For example, everything he writes will be put through an approved* AI tool and then you'll sit down and review it together for the next two weeks. Help to find him some training. Maybe there's a basic writing class at a local college he can take (the IRS says that up to $5250 a year in tuition reimbursement is not considered taxable so that's what most companies offer). There is probably a good LinkedIn Learning course on business writing. See if your org already pays for that or would be willing to cover a month or two of premium for him. If not, maybe he could do some driving for you if you know what I mean. Depending on the size of your org, you might have an L&D department that already has a class for this or someone that might be able to serve as a tutor so you don't have to.
Finally, you have to explain that you're going to coach him, help him, find resources for him, but he needs to step up his game. Decide how long you are willing to deal with this berkeleyery and what level of progress you are willing to consider appropriate and make it clear that right now, it's just coaching and trying to make him better at delighting customers. If the improvement isn't there in X weeks, then your next step will be to drag HR into the mix with a formal PIP. And everyone knows that once you're on a PIP, it's just a countdown timer to having to box up your desk and GTFO.
* Be really really really really careful with how you use AI in this setting. Make sure that you're asking the question of what tools you are allowed to use because just throwing documents into an LLM could result in leakage of sensitive data or data that is covered by some sort of governance. Ask your IT or your infosec department what tools are approved for that use. Including Grammerly.