I'm not sure, in your situation, I would leave the dog outside until he made the decision to come in. It almost sounds as if the dog had bad experiences inside at the previous home and outside was the only place he could stay or maybe was forced to stay; anyway you now have to counteract all that.
All the good points have been made by others, now you have to put them all together.
Do not let the dog outside without a long lead (long closthesline or whatever); when you say come reel in the lead and treat and praise. You have to be consistent too; use the same word, treat immediately, etc. Also I don't think I would leave the dog loose outside until this is overcome. If possible I would also take the time to be outside with your dog (still on the lead) and play with him and praising and treating for any good interaction he does.
Also you might want to have a special treat just for the come command and for entering the house. He does not get this treat until he comes and/or he enters the house - but make it something really special that is not given except for this purpose.
