My LO is almost the same age.... and I've noticed that he nurses MUCH better at nighttime and when we are lying down in bed. So, sometimes, I will go nurse in bed (which turns into naptime) so that he can focus. The other thing that seems to help is nursing him in my carrier, rather than holding him in my lap. For whatever reason, he focuses better. That might be something to consider trying, since you've got the older kids to wrangle?