Will my dog forget me or be happy when I return after months?

That sounds like an awfully big change for Spring, so he may experience some anxiety and stress about being chained up, especially if he hasn't before. Spring likely won't forget you if you return to him, as dogs have gone years before seeing their owners again and are usually happy at their return. Looking into seeing if someone else can watch him that can give him more time, or seeing if a rescue can take him temporarily into foster care may be better off for his mental and physical well-being. Being chained all day long with little exercise or care likely isn't the best for him. You may try contacting a local shelter in your area to see if they have any programs such as temporary fostering that may be a better option.