I started Hollie at 5 months, my hv was dead set against giving her anything before she was 6 months. But she was so ready and was wearing me out feeding every 1.5hours night and day. Just watch for signs, like waking at night when he used to sleep through, milk no longer being enough etc.
The guidelines here are 17 and a half weeks if necessary but recommended to wait until 6 months. My hv did tell me though that if you do start weaning early to do it slowly, whereas if you do wait until 6 months it progresses a lot quicker and you dont have to spend so long on purees and baby rice.