For the first one, the correct anwser would be "Their service at the restaurant was excellent."
Plurals for words like this can be tricky even for natives, and it depends on the context, in this case you're saying the restaurant provided A service, (preparing and serving a meal to a customer), rather than providing services. You could say however that a restaurant providees various services (such as making food, serving food, making and serving drinks, cleaning afterwords). So once again it's dependent on context.
For the second one, it'd be "A man of resources." (or you could say "A resourceful man").
The reason here being what makes him a resourceful man, is his access to multiple resources. Generally speaking I'd say treat resource as a normal noun as far as singular vs plural goes. Maybe if you can give me some more examples I can help you more?
Feel free to message me, I hope this helps!