Family can be so frustrating. We have experienced a similar thing with our DD who just turned 4. The first two read very early and family just expects her to do the same things. One thing I have noticed is that some family members have idealized memories of what the other children were actually reading at that age. Some how the definition has moved from oh the two year old can sound things out there reading to your not reading yet because you can't read everything fluently. No one ever has said this but I realized this the other day when DD4 said she could not read yet because she couldn't read harry potter by herself. There are lots of parts to reading and a set of skills that develop over time. We have now started to make a point of referring to her as a reader and highlighting the things she can read when around family, and then glaring at them until they say wow that's great. Interestingly she has focused much more on her math skills and seems to learn those skill much quicker. I think it is hard for the littlest child because the measuring stick gets set so high, and they unconsciously compare themselves to the older children. Things that completely amazed us with DD1 seem more typical now that we are surrounded by similar children. We have discussed this impact with our family and expect them to be positive about her growth. I have been trying to spend more time one on one with her which is easier with the other two in school now. I worry that she has not gotten as much attention as the other two over the years because she is the youngest.

When I realized this was happening. I asked DD then 3 what she wanted to do about it. She wanted easier books but did not want me to tell anyone about it! I realized we had a shortage of easier books as many of them had been given away or didn't make it through the other two. We ended up stocking her room with early readers and board books. She looks at these on her own time and we only comment on her reading if she invites us to hear a story. She has gotten much more comfortable reading but prefers to interact with the uncles who were also youngest children, both of them seem to really get her. Everyone tries to be very supportive of the kids but sometimes it is hard to be the littlest. Now that you mention it the older two tend to dominate conversations and get there questions taken more seriously by the adults.
I just keep reinforcing that she is loved unconditionally and that her value does not come from what she can do but who she is and that she is my precious baby that I love unconditionally. My DS would roll his eyes at this part when I did this to him. Then I would follow this with lots of hugs and snuggles! Hopefully your family will respect his right to develop at his own pace if you explain the impact the pressure is having on him. If they continue to pressure him I would step in and say something and make it clear that you have certain expectations about the messages they are sending.

I am going to pay more attention to making sure DD4 gets to participate in conversations and that people take her questions seriously. She does kind of get treated like the baby. Good luck with your family. I am curious to hear what happens.