One thing that seems to me to indicate your brother's social/communication challenges is he has Gemini (thinking, also talking) modifying least aspected (strong energy) Venus (relationships) opposite (energy is over-excited by) Saturn (duty, also restriction). This could indicate that it is challenging for him to "connect in" to relating/talking information, which makes the urge to communicate very important, but very challenging, because he is not aware of when he is communicating and when he is not. Since Saturn is the only connection into the rest of the chart, he may feel resticted in his ability to communicate.
And, looking at Mercury (thinking, also talking/daily work - Mercury is ruler of Gemini), it is focused in his 12th house (spirituality, also confusion - Mercury in the house of its fall - very challenging energy), indicating the possibility of confusion in speaking, since the 12th house "dissolves" whatever is in it.
As you have noted, you have strong Mercury (thinking, also talking/daily work) connections, which is especially important given his confusion about daily talking. Interestingly, the strongest, easiest Mercury connections are his Mercury connections to you, indicating he can communication with you. However, his Mars (being, also anger) also squares (energy needs to be combined with) your energies, indicating he could take his temper out on you, particularly on your daily communication (Mars squares your Mercury). In addition, your energy squares his Moon (home, also emotions), indicating your energy could be emotionally stressful for him. Also, his Saturn (duty, also restriction) opposes (energy is over-excited by) your Mars (being, also action), so he could restrict your ability to take actions with him in some way. And his Sun (self-expression) does not connect into your personal energy at all.
As with all relationship situations, the success at your inter-relating with your brother will depend on each of you wanting to relate, to work out the issues between you and your brother. I think it might be easiest to let him communicate to you in his own way, with you replying to his communications. This might lessen some of the stress he might feel if you directly communicate to him (your Mercury square his Moon and Mars). This Mercury connection of his to you may indicate for him a need to be listened to, with you as the perfect listener!
As always in these situations, you have worked with your brother on these issues over the years and so know best how to handle them. These are only some ideas which might give you a new perspective on this most challenging problem.
Wishing the both of you well,