I think his concerns are completely warranted given that the day she met him, she almost got taken to hotel by a 'kind person'. I would trust Shiu to go out alone more than I trust Towa. Treating her like a child? With the way she acts she might as well be one, a grown up looking child, which make it way more dangerous since less people will pay attention if someone tried to do something. While Shiu takes it to the extreme, at least she know the dangers, Towa doesn't. That girl has been shown to be way too kind and far too gullible that if someone less decent met her first, I don't even want to imagine what will happen.
Imagine this based on what we've seen so far, someone ask Towa to help them with something and lure her to a secluded place where there's no one around, normal reaction would be to not be in that situation in the first place since you won't go to suspicious places with strangers but even if you do, alarm bells would definitely be ringing on your mind once you've arrived. What do you think Towa's reaction would be, from what we've seen so far? Would she be suspicious? No, no she won't be the least bit suspicious of their intention.
And just like the other commenter said, while he did forbid her from going out often, once Shiu said that point out loud, he realizes he was being too overprotective and decided to slowly let her learn about society, while he watches over her to make sure she doesn't ended up being tricked by some people. He's not perfect, but he is trying his best and he is actually a good person at heart. An actual controlling person would be furious at her whenever she suddenly goes out anyway, like when she ended up going to the school, or when she goes to his part time job, etc. The guys just a worrywart in my opinion