Is there any problem of somehow tying the esrog to the other 3 minim? this is being done for an older person that has trouble holding the minim together without some additional support.
Answer:
It is not correct to tie the Esrog together with the other Minim (see Rambam, Lulav Chap. 7).
The Aruch HaShulchan (651:9) who explains that the Esrog must be kept separately, and derives this from a Torah verse (whereas the letter vav joins together the other three Minim, it does not appear after the Esrog).
For an older person one should purchase a smaller Esrog and smaller Minim to make them easier to hold.
Best wishes.