1. my son will spend most of the time with me and therefore speaking in Spanish, will this affect his English? This is important to me because we live here and I don't want him to be behind or not be able to communicate properly in this country.
As long as you make sure that he has English influence it should be ok. Send him early to nursery maybe instead of waiting until 3 (if you are a SAHM). Get your OH to be active at bedtime reading, let them have lots of time together at weekends without you.
Go to playgroups and maybe some structured activities where he can meet English children. Maybe some including singing rhymes as this expand his vocabulary.
2. I know I am meant to always speak spanish to him and my OH will only speak English, so what happens when we are all having time together as a family? Dad understands a bit of Spanish but he's def not fluent!
You could introduce English as family time if otherwise your OH's influence is not so much. Or you could "bully" your OH to learn Spanish, this is how my friend managed to get her husband to learn the minority language.
3. What do I do when it is just my son and I in public doing errands, meeting other mums and children for play dates etc.? Do I stick to the Spanish or talk to him in the language everyone else is using?
When I am just shopping, at the library, in a coffee shop etc I speak German. I will also speak German if I just address DD. If I am in a group and address several people I speak English. I gave up saying everything twice.
We live in an area with lots of expats (commuting distance to central London) so it is not unusual to hear lots of different languages.