Any amount of time breastfeeding is better than none. Currently the US recommendation is a minimum of 12 months; people in most other countries nurse significantly longer than that (2-4 yrs and more). Formulas are incapable of providing all the micronutrients and antibodies your body creates. BTW, your body puts priority on creating the healthiest milk possible, and the balance of proteins and fats changes over time to meet your growing child's needs.
Part of the distraction while nursing is his age. This is a normal thing for him to do. The book What to Expect the First Year is an excellent resource.
There is more to nursing to just the nutrient content of the milk/formula. The physical/emotional contact and baby's feeling of security/comfort is different with nursing than with bottle feeding. The antibodies in breast milk will keep him healthier longer, and less susceptible to colds and ear infections. There is a direct correlation with length of breastfeeding time and IQ, improved neuromotor development, reduced infant mortality rates. Nursing toddlers and preschoolers are measurably less prone to allergies, asthma, ear infections, diarrhea, and many other infections and illnesses. Longer nursing duration also reduces risk of insulin dependent diabetes mellitus, inflammatory bowel disease, coeliac disease, lymphoma, and high blood pressure in later years.
Expressing breast milk, whether manually or with a pump, and freezing it in ice cube trays is a common and reasonable way to have milk available for your son when you can't be with him. Start nursing him to get the milk flowing, then express what you can, then let him finish. This method will also increase your milk supply. You can also check with your local La Leche League and local hospital for low/no cost pump rentals, as well as local new mom's groups and Craig's list to obtain used pumps.