Who will feed it and clean the litter tray
I found this comment really odd
, what did you mean by this OP? It will be you, as your DS may end up not being interested in it. So only get a cat if you & your DH actually want a cat & all the responsibilities that come with it!
Cats are wonderful creatures, and please consider a rescue over any others.
When getting a rescue, you usually have to pay a 'fee' (I've paid about £100+ for 2 kittens in the past) which in my experience of getting rescue kittens has covered the cost of vaccinations, & first worm / flea treatment, so well worth it.
There is usually also a home visit involved with rescues, where by someone from the rescue will come and visit you to check that you are suitable to rehome a cat.
Then you will have to cover the cost of micro chipping & neutering (i would probably estimate roughly £80-100 for these)
So I would make sure that financially you are also able to look after the cats needs, as then you have the cost of food, cat litter, flea & worm treatments going forward.
If you were considering rescue kittens, I would get 2 so they have company for one another, but do be aware they are very destructive & will make holes in all your curtains! Also they are very delicate little things, so I'd be wary if your DS is very young.
Please consider it all very carefully first, as too many people rush out & get a cat, but don't fully appreciate the responsibility.
Having said all that, they do make wonderful additions to the family 
Also be prepared to clean up dead mice & birds too if you end up having a hunter!