It's actually quite normal behaviour at this age. 5 minutes is enough time to get all the milk out at this age - but I totally understand your concerns. I was so worried when DS did this about around 4 months.
The reason he doesn't get upset during hte night is most likely because it's dark and quiet and there's nothign much else going on. During the day, however, he's probably getting very interested in the world around him and feeding gets in the way of that. Just note also that if you introduce bottles at this time, he may decide he prefers them (as he can feed while looking around) - just something to be aware of.

Things that helped me:
Find a nice, quiet, dimly lit place to feed.
If he's done, let him go. Don't ever try to force him back to the breast.
Remember it's probably a phase and should pass with a bit of time. Try as much as you can to relax and go with the flow.

If you're concerned about how much milk he's getting then watch his nappies - that's always the best way to tell if he's getting enough. And remember the ABA helpline is always there if you have any concerns or questions.

All the best