It's the province of headcanons, but Holmes seems to be quite an independent and proud fellow to count on his brother. The reason why Holmes and Watson got together was because Holmes couldn't afford lodgings at Baker Street. Surely, had he had (or accepted?) Mycroft's help, he wouldn't have been in the need of a flatmate? Especially considering that both Holmes brothers are extremely private persons. Besides, I'm not sure Mycroft approved of Sherlock's leaving the university after just two years :)