Oh the poor lil chook. I wanna give him a big hug too!

I'd try and partially humour him just for a little while by doing things together, for example if he wants you to open the toilet lid you do one half he does the other. And when he does do it well or by himself (this could take a bit of time to watch him do things) tell him he is so clever, and that you are so proud of him and that it is so good because Juliette will be able to learn from him. Other things of a similar tone, try not to chastise for it because I would think it would continue and then he'll learn to get attention through bad behaviour ie tantrums etc Which is a hard pattern to break when it starts. And try and watch your vocabulary, instead of saying "I can't help you right now." Maybe change it to "I'll be with you in a second." Make sure that you are surrounding yourself with people who don't just make a big deal of him or her iykwim? I had some friends that would go out of their way to give Paris attn thinking she was missing out, but I was worried that would cause problems because then she'd crave it more when those people left. And for those people that are overly cutesy with Juliette, take that time to include him yourself or praise him for being such a wonderful brother etc. Keep reminding him of how lucky you are to have such a wonderful son and brother to Juliette, and when daddy comes home remember to tell him how helpful he has been today etc. Also recap on what the two of you did together, even if its cuddles on the couch watching TV or a conversation you've had. Sometimes its easy for them to forget about the attention they do get so having that chat with daddy (with him present) will help him to see he is still a major priority in your life. Kinda like not taking it for granted iykwim?

I think he's done so well if this is the only signs he has shown thus far, you are doing such a great job He is such a secure confident little man, just think of this as a minor hitch, where he needs a little reassurance as we all do from time to time

OH one more thing, maybe drop the big brother little sister thing for a while, as it could be something he doesn't like hearing iykwim?