Should I approach my boss from my previous company for a LOR for my Master’s in Computer Science (From USA)?
It is likely that your application will be reviewed by a faculty committee. It is likely that some, at least, of them will want to see some prediction of academic success in your letters. If you were applying for other industrial jobs it would be different, but I suggest that at least one of your letters come from someone who knows you well and can make a fair assessment that you will succeed in an academic setting.
A head of a startup or others in industrial positions have a different focus and their letters probably can't properly attest to your academic qualities. It is impossible to judge how much they can help you, not knowing the details, but it sounds like a risky move.
Direct your letters toward the needs of the program you want to join, not the workplace you have been in.