I'm an atheist and a Scout leader in the UK. When writing my programme I always include a spiritual component to it but leave it non-religious. We definitely discuss and explore different religions (in past year visited a Mosque, an Abbey and a Synagogue to learn about their cultures) but never focus hard on a religion or even suggest that holding a religious belief is necessary to be a Scout.
Recently at an event where I renewed my promise I took the Atheist promise which is as follows:
I promise that I will do my best Uphold our Scout values Do my duty to the Queen Help other people And to keep the Scout laws
I find the majority of Scouters I meet, despite saying the original promise follow this non-denominational one throughout their Scouting. We promote the "worldwide family of Scouts" more than a religious grouping.
If you do look for a Scout group for your son, make sure to talk to the leader there. Different groups approach the issue differently so try to find one that suits you and lets your son avoid being 'forcible subjected to the propaganda' though it can be good for a young person to experience it and make that decision for themselves.