and does it fit around their shoulders and how tightly can he be strapped in?

 The straps are similar to a car seat - and you can do them up pretty tight. We've just come back from a 4 hr ride with our daughter (3 yr 3month - 16Kilo) and she's the master escapologist of her car seat now - to the extent that she's worked out not only how to shrug the shoulder straps off but also undo the seat-buckle!

She hasn't been able to do the same in the WeeRide - partly I think because she's sitting immediately under my view - so she doesn't feel she can try and because the buckle sits higher up, on her chest (on a car seat it's right down at her groin) there's actually less space to move the straps around and slip a hand through!

Does that make sense? - I fear I'm not describing it very well. Finally of course - there's the fact that your arms and body would be around him - so by it's very nature - it's secure!