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.
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.