1. He is most strong.
2. He is a most strong boy.
3. He is the strongest boy.
4. He is strongest.
5. He is the strongest.
(Are they all grammatical? Does 'mose in #1 or #2 mean 'very'? Which one is correct, #4 or #5? Are both the same in meaning?)
In #s 1 and 2, change "most" to "very" and they will be fine.
#3 is correct.
#s 4 and 5 need to have a noun after "strongest."