Hello there.
Unfortunately, to my knowledge, there is no program that can convert regular code into code blocks - that goes for any language, not just Java.
What is it that your son finds stressful about learning Java? Is it classes/objects? The syntax? The tools that you use for it(compiler, IDE/text-editor, etc)? Asking him about this stuff helps you understand the situation better and will help you work on helping him learn it. It's important to note that Java is a gigantic step up from Scratch: it's object-oriented(which is more complicated than any procedural-language) and has a somewhat complicated syntax(i.e. in the middle between a simpler language like Python and a complex language like C++).
In the event that even after talking to your son and helping him that he still cannot learn and understand Java(or any computer-science concepts for that matter), I'd recommend stepping down to something simpler(but not Scratch). Give Python or Lua a try and see what happens then. They contain a simpler syntax and are easier to learn and work with. Although neither language is used in Minecraft, there will be language-bindings/mods that allow support for them in Minecraft. For other games, Lua is supported in Roblox and GMod - both are quite popular. Python is less used for games, so you might have trouble trying to find a game to put it to use in(but your son could always build his own game).
I hope this helps.