Holiday planning!

Hi.  I'm new to this so apologies if you get this sort of question all the time...Our 6 year old son was diagnosed with ASC 5-weeks ago. Holidays with him have been hit or miss over the years.  I'm trying to plan our summer holidays and looking for recommendations for anywhere in Europe with nice weather (but not too hot - he hates the heat!) that people have gone to that might suit a 6 year-old with fairly high functioning ASC and a bright, sociable, very physically active 8 year old son. My ASC son is good in restaurants and loves the sea and nature but doesn't like crowded hotels, pools and beaches (nor do we!), so we tend to favour places with their own pool (which gets v pricey!) but near to small towns etc.  We also like to sight-see in small bursts.  He loves museums and churches.  We are looking to spend an average amount of money if that's any help! Any thoughts?