“Tom is the strongest of them”

Question

So there is a picture of three boys lifting weights. Tom is lifting the heaviest one.

"Tom is the strongest boy of them."

Is this sentence correct in describing the picture? And if not what exactly is the reason it is not? How about

"Tom is the strongest of them."

Adding the word 'all' to the end of the sentence seems to fix the problem to my ear but I'm not sure the grammatical reason of why that is.

Answer 1

Since you're using the superlative, you don't technically need any qualifier. 'Tom is the strongest' is sufficient.

Answer 2

"Tom is the strongest boy of them."

seems odd in that they are all boys, so the additional qualifier seems odd. Now, if that was a group of mixed gender, that might make sense, in qualifying that one of the girls is still stronger that Tom - otherwise this redundancy is redundant.