Client frequently paying late. Manager claiming I make mistakes but doesn't prov...

I work for a small MSP. The owner is almost always late paying my invoices and I usually remind him a couple times. In addition to the invoices, he requested I fill out time sheets in the proprietary system he built. Recently he said that my invoice doesn't match the time sheets so he fixed the time sheets for me but wont do this again. I'm not sure what the mistakes were but I actually copy the data from the time sheets to my invoices. What should I do to have him pay on time? Some of my ideas are

1. tell him that he has a certain number of days to point out any errors/inconsistencies but he can't withhold payment for over a week without saying anything
2. charge a late fee for late payments
3. suggest that we only use invoice or time sheets as they seem redundant
4. suggest I can test and fix bugs in the time sheet system he made
5. suggest that if something looks wrong or is inconsistent, he sends me a screenshot and documentation

I've actually noticed a bit of a pattern of him claiming I made a mistake and when I provide him evidence I didn't, he drops the subject but doesn't concede. Another time he told me he reset a password that I needed to use. He said he recorded the new password in a particular system. This system was big and I sent him screen shots of all the logic places I thought he would have saved it. I don't think he even looked at my screenshots and just told me it was there in the same place I sent him a screenshot of. Finally he sent me a screenshot with the password in the system, I think he has view that is different than mine when logged in. I had already messaged him that this may the what was happening and he just didn't respond to this comment, and it seems to me this is an unresolved technical issue.

You are describing red flags of a business that is likely poorly run or doesn't have enough money to pay their bills.

You didn't say if you are billing monthly or weekly, etc. If you aren't billing weekly, consider insisting to go weekly. Use the "if there are any questions we can address them quickly" as a reason. It limits how much money you'll lose if or when they stop paying.

A business that chronically doesn't pay on agreed terms is either sloppy, or doesn't have money to pay. Sloppy can be fixed if they want to. But, homegrown systems for AR/AP isn't the best use of a small tech company's time. There are way too many inexpensive options that do a great job for that.

So - homegrown timetracker is a second red flag. If a business owner really thinks their time is better spent maintaining a homegrown system than focusing on getting and servicing clients or learning new tech is really running a hobby not a business. Before anyone DVs over this statement - the excuse of creating the homegrown system is learning new tech doesn't hold water.

As a contractor (at least in the US), you do not have anywhere near the rights of an employee.

If you are hoping that someone can give you ideas how to get this person to pay on time, that isn't going happen. They either appear to not have the resources or not the respect to pay their bills.