My partner has on his bike 700 x 40c tyres and he's not finding them comfy to cycle on. Is it possible to change to thinner road tyres and if so would the sizing still be the same or would he need a smaller size ?
Thinner road tyres will only add to the discomfort, 700 is the wheel size and 40c is the tyre diameter, anything less than 40 and you are going to feel more of the road surface, its a trade of between comfort and rolling resistance.
I started out using 35mm tyres at 50 psi, your body needs to acclimatise to cycling, I still use these tyres on one bike and run them at 85 psi(the upper limit) now.
But the first thing to do is to get any bike properly set up for the person. The saddle is first and this will work best within a small range of positions. Height and distance back from the pedals is the first thing then getting the angle of tilt afterwards. Then the fit works forward to the bars, or the reach. Bar height can be varied, it may start higher and drop as fitness and flexibility improve, but the reach will be pretty constant. If you don't do this is can be a pain and cost extra, I messed about with bars and stuff first then I got the saddle right. Of course my bars were now wrong and so I'd wasted money on new parts that no longer fitted.
A good bike shop will help with all this, the position probably free of charge. Also people complain of a bike/saddle being only comfortable for a certain distance. Sheldon Browns website points out that a lot of us slump as we tire, thus posture gets poor and a previously comfortable setup feels harsh.
I'd do all this forst before spending money on tyres, wheels, etc.