Physique is the physical shape of your body. So a strong physique is to have a muscular/powerful body. A strong constitution, is to have a strong resistance to illness and disease. i.e. not get sick easily and if you do get sick to fight it off quickly.

They are synonyms. They can be used as the same word, just depends on the context of their use.

We do not normally use those words in that manner. Usually, "he has a [good] physique," or "he has a strong constitution." Physique is not 'strength,' it is appearance. It can imply strength, but we usually don't say physique is strong in American English. Roy

in the context of the statement 'he has a strong physique' - this is generally talking about physical body, build, musculature of the subject. in the context of the statement 'he has a strong constitution' - this is talking about the mental faculty, willpower, resolve of the subject. i.e.. nothing to do with physical body.