Physically, he is in his prime, and will continue to develop heavier bone, a deeper chest, a fuller coat, as well as greater strength and endurance. Large breeds attain their full physical maturity and definitive proportions between the ages of 18 months and 2 years, while smaller breeds reach their maturity a little sooner.
An adult dog no longer needs a body-building diet, but more of a maintenance one that is adjusted to his daily energy requirements. Too much or too rich a diet will lead to obesity and perhaps more serious health problems.
Psychologically, he is unconditionally devoted to you and may become jealous and possessive. He may even begin to resemble you in some ways. His intelligence and receptivity are at their peak, and he vastly enjoys collaborating with you in work and play, as well as acquiring new skills. He knows and understands you and your habits as well as you know and understand him. These should be the most harmonious and rewarding years of your life together.
What he needs most is training for work, sport, or merely for fun and to exercise his intelligence, lots of mental and physical activity, mutual loyalty and devotion from a loving owner.