First of all i would like to both and your son for the great score in jee main .

sir , you have mentioned that your son got 98.04 percentile so it can be assumed that it will give him a rank around 17000 which is not bad at all. There are also some student who will go for iits so it is sure that he must get a sit in the nits. But whether the NIT will be good or bad and the branch he will going to get is completely depends on his state rank . It may be happen that with his rank he can get a sit in the NITS of your state in a good branch or he can get a branch that is not preferable to you. With this rank if you think to take admission to those NITS that are not situated in your state then let me inform you that in NITs 80% sits are reserved for the state domicile students.

ALSO,

There is a matter that only  jee main January has held and it will held one last time in this year . If he can not perform well in the next attempt then the percentile  will decrease as there is lots of student who overtakes him and that will big trouble for him.so , i am saying that january jee mains does not plays an important role for getting sits in NITS as they consider only the best of two attempts. so according me , Your son should focus on the next attempt because that will decide every thing. If in the next attempt also get 98.04 or something like that then there is a huge chance to get into NITS.

BEST OF LUCK FOR YOU AND FOR YOUR SON .