Why won't my boyfriends parents let him stay over at mine?

ok so i've been with my boyfriend for about 3 years now. we love each other so much and we're really happy together. we have always lived about 40 mins to about an hour away from each other so we usually only met up once a week. we now go to the same uni & get to see a lot more of each other during the week which is great but during the summer and holidays it goes back to once a week. the problem is that organising a way to meet up can sometimes be difficult and sometimes we don't get too long together. I've suggested that he stay over at mine for a night or a few days even so we can have a bit longer together and have more time to go places together like the cinema or even a night out which is harder to do when its only a visit for the day. He always replies that his parents are never too keen on the idea and he says that they are very old fashioned like that. I would understand that he wouldn't have much choice when he was younger but we are both 19 now nearly 20 and it's frustrating that he still seems to have no freedom. I see other couples stay over at each others or go away on holidays with each other and that is also out of the question because of his parents. My parents also don't understand why they are so against this as it would make things a lot handier sometimes. It just feels a bit unfair & i wish he would try a bit harder to open their minds a bit but i feel like he will just do what ever they say no matter what. I feel like if he doesn't speak up a bit more for the things he wants to do it'l always be like this. Another thing that is strange is that i have only been to his house once for his formal and it's always my house that he visits. Again i think this is something to do with his parents. I'd just like to know is this common for anyone else at this age & is there anything i can do to change it or persuade my boyfriend to try harder with them so he can have more freedom?