Welcome to the SI101 Board and its Forums. I hope you enjoy participating. Please begin by familiarizing yourself with the site and reading the FAQs, the Posting Guidelines section, and especially the Index, all found at the top of the main screen. You can also do site searches.
It is unnecessary to open your mouth wide. It is only necessary to open it wide enough for his penis to slip inside between your moisten lips. Your lips should wrap around your teeth so you do not scrape the skin of his penis.
By not opening wide or as far as you can, you will not tax and thus tire your jaw muscles. So, drop or relax your lower jaw and let him slip inside. If this does not work for you, then try limiting your excursion to only the Glans. This is the most sensitive area anyway and you can lick/lap the Frenulum {Y-shaped ridge of skin on the underside extending from the cleft to the top of the shaft).
Please read the following article listed in the Index:
How Do I Get Him/Her to Orgasm From a Hand/Blow Job?
Give this a try and report back. I hope this is of help.