Hello there my son also is 5 and has a 2.94 cm ascending aorta with other involvement. I'm not sure if this is allowed (Admin let me know!) I'd like to invite you to a group Aortic Dilation in Children on Facebook. There are other BAV and Marfan moms there, some that have had success with losartan and also a cousin of losartan. I'm going to send you a Message!
Bicuspid aortic valves are associated with abnormal aortas, called an "aortopathy" in some patients. The ascending aorta can dilate to various degrees. This can occur with or without stenosis of the aortic valve. Fortunately the incidence of aortic dissection or tearing is lower in these patients than it is in patients with Marfan syndrome. Unfortunately we do not know what the true incidence of adverse outcomes is with the bicuspid aortic valve population, so all we can do is extrapolate to patients with Marfans (a much different disease). That being said, what is most important is the rate of change in the size of the aorta on an annual basis. Surgery to replace the aorta is generally not performed until 4.5 to 5 cm in size; or if the aorta grows by 1 cm per year. Medical treatment to try to impact on this condition is still unclear. We do not know if losartan or any other medication really changes the course of this condition. Generally serial follow up and avoidance of sports where there could be contact to the chest, or stressful activities that make you strain, like wrestling, weight lifting etc, is all that is needed in childhood and adolescence. I agree with your decision to get a second opinion.