Autistic 8 year old boy has brother (5 years) with speech impairment, their cousin (6 years) is hyperlexic. do you recommend any genetic testing?

No: There are probably genetic factors in all of these, but these are polygenic. Hyperlexic kids often do very well in life. You won't find a single locus mutated that will inform a decision about preventing more children like this from being born. Genetic testing is for major, catastrophic illnesses like cystic fibrosis or muscular dystrophy -- single-gene diseases.