First congrats on getting the seed to sprout and keeping your Joshua tree alive for five years in a container! This plant is a yucca (Yucca brevifolia) and thrives in the unique climate of the desert, characterised by dry air, cold winters, and extremely hot summers. It does not tolerate humidity well, making it difficult to grow indoors. I suspect the yellowing leaves are a sign of excessive watering. If you have any doubts, please refrain from watering. The soil needs to be extremely dry before adding a cup of water.  Even watering once a month could be too often.   The two new shoots, plus the green in the image you sent, tell me your Joshua tree is going to survive. Try moving it to the hottest spot outdoors this summer so it can experience some summer heat. This plant needs little fertiliser, but after growing in a pot for several years, it may respond well to just a small amount of slow-release plant food.  I would suggest 1/4 teaspoon of Osmocote sprinkled onto the soil. Do not grow near a sink or in a bathroom, as the humidity may be too high, causing the foliage to rot. The good news is your son should be enjoying his namesake plant for years to come. The Joshua tree is a long-lived plant as long as you remember to keep it dry, give it sun and don't fertilize very often. Keep growing, Marianne Binetti