My local-ish craft shop does needlecraft classes for children (Suffolk if you're around here). Maybe you have similar not too far away?
I'd think about the John Lewis machine for £49 as it'scheap simple and probably a good starting place without all the gadgets. Maybe see if someone's upgrading and selling theirs on ebay or similar. link
Sorry I haven't been back , I have been looking at the machines recommended and tbh I am a bit nervous about the safety aspect . I think I am going to try and put him off this year and look at other things .
I am going to get him some hand sewing kits for now .
If I was handy with a machine I would probably not worry so much , but its not my thing .
Don't give up. I let my seven year old use an old hand crank sewing machine. By turning the hand crank they are fully in control of the speed of the needle and so are less likely to sew through a finger-she hasn't managed it yet! But it gives a proper stitch which is durable and is fairly quick. I have sewn a dress and apron on it and quite like using it for small jobs. Look for singer ones on ebay-don't go for a treadle on and try and avoid a bullet shuttle one (I think 15k type) as they are a bit tricky to thread the bobbin. If you live near Southend I could talk you through it but I think it is a really good idea for him to have a go and I would not pay over £35 for one-I reckon this one should go for about £43 ish as it has a lot of stuff and the original booklets. www.ebay.co.uk/itm/Vintage-SINGER-Sewing-Machine-Attatchments-/200916884566?pt=UK_CraftsCollect_SewingMachines_RL&hash=item2ec7945c56