Hi muru, and welcome!
1. As we live within catchment for DCGS, if he gets 121 in Bucks test will he definitely get a place in DCGS?
Yes, quite definitely.
If so do I still have to put RGS in the list or shall I take it off from the list?
You can take it off the list.
2. If he doesn't get 121 in Bucks, what is your advise between St Clement Danes and one of slough schools? We like SCD also and from the historical data he qualified for 10% academic place. But as it is not a grammar school I am bit confused. Do I have to put Berkshire school also in the list?
I can't help you on the Slough schools, but as they are fully selective, whereas SCD is not, it is likely that the results will be higher at the Slough schools.
Have you looked into transport from Amersham to Slough, though? The journey time can be as much as 90 minutes.
3. Amersham school is just 5 mins walk from home. But Chalfont community college got better ofsted report than Amersham school. I need your expert advise between these 2 schools.
CCC probably does have the edge, but Amersham is a rapidly improving school. Have you visited either of them to make your own judgement?
You must definitely put Amersham on your list though, because your DS might not get a place at CCC.