Hi, Welcome to HealthcareMagic Forum.
It is absolutely normal for the veins to Dilate, that is to enlarge, after a Very Hot water shower, which appear as Bluish lines, because of the increased venous blood
return. There is nothing to be concerned about. See to it that next time he has a shower, the Water is warm and not too hot.
Since he is not too much active, may be you should be taking him to a Pediatrician
so that he can examine him and run few tests to check his Hemoglobin
levels. May be he is malnourished, this has to be ruled out. Give him Lots of fresh fruits and Green leafy vegetables. He needs a Protein rich diet which should include, eggs, chicken, Fish, milk. See to it that he eats and sleeps on time and limit his TV viewing duration. He should be avoiding Junk.
I Wish him Good Health.