This is an extremely good percentile, first of all I would like to congratulate you and your son. The ranks are not known completely but we can give a rough and approximate figure about the ranks. His rank would be something around 11 thousand to 12 thousand roughly. 

Sir/Ma'am since JEE Mains Examination is happening twice this year in January and April in various shifts on various days the exact rank cannot be estimated. For percentile 98.679 the approximate rank will be near 11558(+-). Note that this is just an estimation and the rank can change after JEE Main is again conducted in April depending upon number of students appearing for the exam. 

Ma'am/Sir if you have any other queries do mention it down in the comment section. 

Your son got excellent percentile score.As this is percentile score it only tell about relative performance of any candidate in particular session in which he/she appera for exam.I can give you rough idea about your son's rank.You can use this formula to estimate rank :Probable Rank= (100- NTA percentile score) X 874469 /100.Probable Rank of your son's =11k.This is predicted rank.

Wait for exact rank till NTA decalres raw score and cutt off score .Score and ranks will be decalred after 2nd phase of jee mains exam.

You can ask if you have any other question.

GOOD LUCK!