Hi! My suggestion would be to reinforce him greatly whenever you can! For instance, if you let him out to use the restroom or to play, he is likely really enjoying his time outdoors and would rather stay out than go to you, I like to do a surprise jackpot consisting of about 5 pieces of kibble or just a special treat. But if he really isn't coming at all, try working close to him at first then building distance. Then after you've mastered a reliable recall at a distance then you can work more on distractions but remember to temporarily decrease distance while working on distractions!