What is the best suggested way forward with my tech startup?

Hi
This is indeed not an easy situation and I can understand your frustration. I could mention the various ways you could have avoided this, but this wouldn't be helpful at this point, so I'll skip it. Just please note that next time you develop something, talk to me before so I can help you prevent this situation.
What CAN be done:
1. Send him one last email with both a 'carrot' and a 'stick' - meaning that if he gives you all the updated code you'll pay him X (do not pay anything upfront) (this is the carrot), and if he does not offer the code that you plan have spoken with a large private detective firm in China who also does cyber crime and you intend to invest your last remaining amounts on finding him and bringing him to justice for breach of agreement (before sending the email, make sure that you have backed up all copies of the code/materials and changed all passwords).
2. If you tell me more about the service/app, and if I think it has value, I can connect you with another company which might be willing to complete it for you in exchange for shares in the company. You would only have to pay for the drafting of the legal agreement.
3. Find another tech-co-founder who can complete it for you. There are many platforms for finding co-founders. Try look online, or during a call I could provide you with the information.
Good luck
I've successfully helped over 300 entrepreneurs, startups and businesses, and I would be happy to help you. After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david

There is no way to determine differences in your code + his code.

Suggestion: Avoid working with people outside your own country of jurisdiction/domicile, because anytime another government goes south, you can lose your entire development staff.

I've had many clients who have lost small fortunes with Chinese + Russian developers simply disappear.

Consider hiring a developer to audit + document your code, so any developer can take over the project at any point.

you should get to as many supporters as you can. they will give you funding and support your ideas.