I always believe that the usage of so and too is different from each other:
He is so busy until he is able to forget his problems. (=+ve sense)
and another one
He is too busy until he does not have time to eat.(-ve sense)
Consider the following sentence:
i)This place is so hot.
ii)No wonder, the shirt's so big.
How about the sentences above?
Is it grammatically permitted to use them interchangeably?
The error you have made is that "so" has two different uses. One is an adverb which has a similar application as "too", for example:
That is so kind of you.
That is too kind of you.
However, the meaning is not identical and they are not strictly interchangeable. "So" means "to a great extent" whereas "too" means "to an excessive degree" (although some colloquial expressions break this rule).
In your examples, it seems like you intend to use "so" as a conjunction, although you are not using it correctly:
He is so busy until he is able to forget his problems.
It seems that you are trying to say he forgot his problems because he was so busy, in which case it should be phrased as:
He was so busy that he forgot his problems.
