What is his role during the jams? Does he lose interest because he is just playing the same 2 or 3 chords for long periods of time?
Is he more inclined to play a heavier style lead that relies more on thick dirty tones than melody and harmony?
For rythym/accompanying/Bobby I would encourage him to look into creating his own melodic sequences with 2 or 3 note voicings during jams, perhaps as counterpoint to the lead player. This can take a long time to really commit triad/2-note voicings of 7th chords and inversions to memory well enough to be able to improvise with them. I find it takes regular exercising of them or else I quickly forget them too, especially their relationship to each other. But when I have a good command of this type of comping it can be more enjoyable/satisfying than lead improv. Probably best to really master a few voicings at a time in one scale/key, apply them to applicable songs, and then start with a few from other scales/keys (minor vs. major vs, mixo etc.). It can be much more satisfying if there is a keyboard player to fill in some of the space left, especially at first. Finding ways of complimenting each other, no matter how small the contribution sounds on its own, rather than doubling and tripling the same notes among bandmembers. Its about the sum total. Its tough not to over-focus on one's own playing sometimes, and the desire to play something really amazing in its own right.
Is it the extended improv that he isn't comfortable with or the songs themselves? For improv I try and remember to take it easy and not try to force things. This is another reason it is nice to have accomplished (but humble) players at all spots...so when ideas/inspiration aren't flowing, there are others chiming in with ideas to play off of and fill in the gaps. Makes the music as a whole better too. There is often moments of feeling lost in improv jams like the Dead do, becoming comfortable with this can be difficult for some folks who are used to the instant gratification of more straight ahead jams.
Communication and sharing examples of what each others current likes are, and ideas on how to compliment the other's playing/goal is good too. The meandering, relaxed, adventurous type jamming the Dead did isn't for everybody, especially at first. It took the Dead years to find their own roles solidly and to have confidence in them. I find it takes a great deal of command, versatility, and chops, but concurrently patience, humility, experimentation, and an extraordinary ear to listen to others, combined with the courage to jump in and the capability to pull off novel ideas on the fly. It can really trick me into feeling unsubstantial as an individual player at moments, become aggravated I cant pull off what I am hearing in my head, or into being extra critical of others playing. Remember its about the whole bands relationship cumulatively, its not about showing off one at a time like a lot of rock music.
