You certainly can do it yourself: I helped my DD a couple of years ago (Birmingham and Walsall tests, similar process and chance of success as Gloucestershire - I have friends there too who self-tutored.)
You will need to invest in some workbooks: we found CGP the best, but it's worth trying a bit of material from couple of other publishers towards the end, just for variety. You can sometimes get pre-used workbooks cheaply if you join a local Facebook 11+ group, as people tend to sell their books on once they have finished with them.
I know of a couple of people who used Atom, and it does seem good, but it's not cheap so if you're watching the pennies you don't really need it. We didn't use it. However, we did invest in entering DD in for a couple of mock tests during the summer, which provided invaluable experience of test conditions.
Our DD was bright like your DS, but more importantly, she really wanted to go to grammar school and was very compliant and straightforward to work with. We didn't have arguments at all, beyond a few 'do I have to do it now?' moments (she did 20 mins before school four days per week in Y5, and slightly less in the summer holidays as she seemed ready). If you found home schooling stressful, I would think hard about how to make the 11 plus prep as low stress as possible. A grammar school is not worth damaging your relationship with your DS for.