I was wanting to know if omega 3 is ok to give to a 5yr old w/adhd?

my doc said its ok. but the pharmacyst said no because it can cause diareha and some other side effects and he just doesnt recommend it. i did some research about it, and have not yet seen anything backing up his input. all i read was people saying that it helped them focus and relax...and thats what its supposed to do. support the nerves, etc. in the brain...not to mention other helpful things. so who should i listen to? i also went to a nutrional food store and they actually agreed on him being able to take it. they had it in childs gummy form and a gel tab.