Hello and welcome to the forum.
Alfalfa hay has a higher calorific content than timothy. I would continue just with the timothy, as you want a steady rather than rapid weight gain. There are several types of pellets and Excel is recognised as a good make. As the breeder has probably told you, it's best to continue with the same type of pellet that the bunny has been eating so far. If you wanted to try a different make, then this must be done gradually over time, so that it doesn't upset his digestive system. Here is a link to a chart, which compares the nutritional qualities of the different pellets
https://www.therabbithouse.com/diet/rabbit-food-comparison.asp
Any further questions, then please just ask.