There is an easy way to check the progress.
Where is the source code? If he worked on the backend he can show some source code. In addition, ask him to push the source code to a tool like bitbucket.org. It is free for a small team (github requires to pay even for a single private repo). The benefit: you can see all the changes, step by step. And when they have been done.
If he is not serious, he will say he doesn't use source version control systems. Maybe he argues because of the low budget and the time. But actually there is NO reason NOT to use it. git/hg/svn is absolutely necessary for every developer and every project. No way round.
That said, he might also argue he is using SVN and not git/hg as supported by Bitbucket. There are several SVN hoster out there to which he can move the repo.
Don't be afraid: even when you cannot read source code, you can identify changes with these tools. They all provide web interfaces where you can login and see what happened. If you have a friend with basic programming skills, he can easily spot changes in an instant and tell you if these are fake changes or real implementation work.
If he doesn't share the source code with you, he most likely don't have one. Tell him that you'll only send out another payment until you can see some progress on the source code.
He is right at one thing: backend sometimes takes its time. But this does not reflect in the source code. It is probably the case there is nothing really useful you can show to a customer. But in source code there must be changes.
Another point: even when there is nothing "useful" to show: you should be able to see some features at specific point. For example, you should be able to see a working login after f.e. 1 week. If he says he needs to ramp up the framework first you should see some features after 3 weeks. To see absolutely nothing is very unlikely.
Besides the source code control you can use tools like trello.com or agilezen.com or kanbanery.com etc. In that tools you can list all the task which need to be done. The developer should even put technical things there, like: "installing database", "installing framework", etc. When a task is done the task card needs to be moved to finished. If that happened, changes in the source code are visible.
My tip: immediately take back control of your project. If necessary, hire a second, independent developer for a few hours to look at what has happened so far.
