Getting him to recall off squirrels is the pinnacle of recall for your dog. So you need to work on recall and engagement with you in easier situations to start with. If you can I would forget about the recall for a while and concentrate on other training things he can achieve and will directly help with recall. Having dogs should always be fun and sometimes the frustration can get in the way of that 
General games will really help with this and can be added to your recall toolbox.
I would start with a hand touch. Show your palm to the dog and click and treat as he moves towards it. Over a short time you can add the word touch and you can put you hand in the air or near the floor and the dog will come into you to touch your hand. Dogs love this and it can be used as a fun way to recall over a long distance.
Teach a middle or peeka boo. Teach your dog to sit between your legs. This again is a fantastic game for a recall. Calling your dog from a distance and into this position is fun and can be rewarded with a tennis ball chase from the middle position.
Teaching a stop is great fun for most dogs. I do this by having the dog facing me and then put my hand in the air with food in it and throwing the food behind the dog - the dog learns that good things happen behind it so will stop quickly.
There are many more - if he is a collie cross will he play with a tuggy? This can be great fun. Introduce a few ground rules - he can only take the tuggy with a get it command and yo need a good release or stop playing tuggy command - do this by saying "thank you " and putting a treat near his nose.
If he gets on with the tuggy you can progress to a flirt pole and this is amazing for teaching impulse control around squirrels etc. Basically it is a furry tuggy on the end of a long lunge or whip line. You can use your get it command and the dog is released onto the flirt pole. You will build up to teasing the flirt pole and having the furry tug move but your dog will only release onto it with your release command. This is great fun for your dog and you and will make recalling from the squirrels easy.
It would be good during this time to prevent as much squirrel chasing as possible so agree beach walks would be good - the more the dogs get to practice the chasing the better they get at it!
All these are different from a traditional recall but will add fun and enjoyment to the same response. If done enough your dog will be listening out for you and will it will be more instinctive to react immediately to you as this means the fun will start not like before when recall meant the fun thing (chasing the squirrel) had to stop.